The Vertebrate Skeleton

• Group Ciconiiformes.
• Subgroup Steganopodes. Sula—gannet.
• Pelicanus—pelican.
• Phaëthon—frigate bird.
• Phalacrocorax—cormorant.
• Subgroup Ardeae. Ardea—heron
• Subgroup Ciconiae. Leptoptilus—adjutant.
• Ciconia—white stork.
• Group Anseriformes.
• Subgroup Palamedeae. Palamedea }
• } screamers.
• Chauna    }
• Subgroup Anseres. Anas—wild duck.
• Anser—goose.
• Plectropterus—spur-winged goose.
• Cygnus—swan.
• Mergus—merganser.
• Group Falconiformes.
• Subgroup Cathartae. Cathartes—American vulture.
• Subgroup Accipitres. Falco—falcon.
• Vultur—vulture.
• Harpagus.
• Gypogeranus—secretary bird.
• Group Tinamiformes.
• Subgroup Tinami. Tinamus.
• Group Galliformes.
• Subgroup Galli. Gallus—fowl.
• Pavo—peacock.
• Subgroup Opisthocomi. Opisthocomus—hoatzin.
• Group Gruiformes.
• Gruidae—cranes.
• Group Stereornithes. Phororhacos.

• Group Charadriiformes.
• Subgroup Limicolae. Charadriidae—plovers.
• Parra—jacana.
• Subgroup Lari. Laridae—gulls.
• Alcidae—auks.
• Subgroup Pteroclidae. Pterocles—sandgrouse.
• Subgroup Columbidae. Columbae—pigeons.
• Didus—dodo.
• Pezophaps—solitaire.
• Group Cuculiformes.
• Subgroup Cuculi. Scythrops.
• Subgroup Psittaci. Stringops—owl-parrot.
• Group Coraciiformes.
• Subgroup Coraciae. Coracias—roller.
• Buceros—hornbill.
• Upupa—hoopoe.
• Subgroup Striges. Owls.
• Subgroup Cypseli. Cypselidae—swifts.
• Trochilidae—humming-birds.
• Subgroup Trogonidae. Trogons.
• Subgroup Pici. Rhamphastos—toucan.
• Picus—woodpecker.
• Group Passeriformes. Crows, finches, larks, warblers,
• and many others.

C. MAMMALIA[18].

Class MAMMALIA.

• Subclass (I). Ornithodelphia or Prototheria.
• Order. Monotremata.

• Family Ornithorhynchidae. Ornithorhynchus—duck-bill.
• Family Echidnidae. Echidna—spiny ant-eater.
• Group Multituberculata. Tritylodon.
• Subclass (II). Didelphia or Metatheria.
• Order. Marsupialia.
• Suborder (1). Polyprotodontia.
• Family Amphitheriidae. Phascolotherium.
• Family Didelphyidae. Didelphys—opossum.
• Family Dasyuridae. Thylacinus—Tasmanian wolf.
• Sarcophilus—Tasmanian devil.
• Dasyurus.
• Family Peramelidae. Perameles—bandicoot.
• Choeropus.
• Family Notoryctidae. Notoryctes—marsupial mole.
• Suborder (2). Diprotodontia.
• Family Phascolomyidae. Phascolomys—wombat.
• Family Phalangeridae. Tarsipes.
• Phalanger—cuscus.
• Phascolarctus—koala.
• Thylacoleo.
• Family Diprotodontidae. Diprotodon.
• Family Nototheriidae. Nototherium.
• Family Macropodidae. Macropus—kangaroo.
• Family Epanorthidae. Coenolestes.
• Subclass (III). Monodelphia or Eutheria.
• Order 1. Edentata.
• Family Bradypodidae. Bradypus }
• }—sloths.
• Choloepus }

• Family Megatheriidae. Megatherium—ground sloth.
• Family Myrmecophagidae. Myrmecophaga—great ant-eater.
• Cycloturus—two-toed ant-eater.
• Family Dasypodidae. Chlamydophorus }
• Dasypus }—armadillos.
• Priodon }
• Tatusia }
• Family Glyptodontidae. Glyptodon.
• Family Manidae. Manis—pangolin.
• Family Orycteropodidae. Orycteropus—aard vark.
• Order 2. Sirenia.
• Family Manatidae. Manatus—manatee.
• Family Rhytinidae. Rhytina—Steller's sea-cow.
• Family Halicoridae. Halicore—dugong.
• Family Halitheriidae. Halitherium.
• Order 3. Cetacea.
• Suborder (1). Archaeoceti.
• Family Zeuglodontidae. Zeuglodon.
• Suborder (2). Mystacoceti or Balaenoidea.
• Family Balaenidae. Balaena—right whale.
• Megaptera—humpbacked whale.
• Balaenoptera—rorqual.
• Suborder (3). Odontoceti.
• Family Physeteridae. Physeter—sperm whale.
• Hyperoödon—bottlenose.
• Ziphius.
• Mesoplodon.
• Family Physodontidae. Physodon.

• Family Squalodontidae. Squalodon.
• Family Platanistidae. Platanista—Gangetic dolphin.
• Inia.
• Pontoporia.
• Family Delphinidae. Monodon—narwhal.
• Phocaena—porpoise.
• Orca—killer.
• Globicephalus—Ca'ing whale.
• Grampus.
• Lagenorhynchus.
• Delphinus—dolphin.
• Tursiops.
• Prodelphinus.
• Order 4. Ungulata.
• Division A. Ungulata vera.
• Suborder (1). Artiodactyla.
• Section (a). Suina.
• Family Hippopotamidae. Hippopotamus.
• Family Suidae. Sus—pig.
• Babirussa.
• Phacochaerus—wart hog.
• Hyotherium.
• Family Cotylopidae. Cotylops (Oreodon).
• Cyclopidius.
• Family Agriochoeridae. Agriochoerus.
• Family Anoplotheriidae. Anoplotherium.
• Section (b). Tylopoda.
• Family Camelidae. Camelus—camel.
• Auchenia—llama.
• Section (c). Tragulina.
• Family Tragulidae. Dorcatherium (Hyomoschus)—chevrotain.

Section (d). Ruminantia or Pecora.

• Family Cervidae. Moschus—musk deer.
• Cervus—deer.
• Cervulus—muntjac.
• Hydropotes—Chinese water deer.
• Family Giraffidae. Giraffa—giraffe.
• Sivatherium.
• Family Antilocapridae. Antilocapra—prongbuck.
• Family Bovidae. Tetraceros—four-horned antelope.
• Gazella—gazelle.
• Bos—ox.
• Bison.
• Bubalus—buffalo.
• Suborder (2). Perissodactyla.
• Family Tapiridae. Tapirus—tapir.
• Family Lophiodontidae. Lophiodon.
• Hyracotherium.
• Family Palaeotheriidae. Palaeotherium.
• Family Equidae. Hipparion.
• Equus—horse.
• Family Rhinocerotidae. Rhinoceros.
• Elasmotherium.
• Family Titanotheriidae. Titanotherium (Brontops).
• Palaeosyops.
• Family Chalicotheriidae. Chalicotherium.
• Family Macraucheniidae. Macrauchenia.
• Division B. Subungulata.
• Suborder (1). Toxodontia.
• Family Astrapotheriidae. Astrapotherium.
• Family Nesodontidae. Nesodon.
• Family Toxodontidae. Toxodon.
• Family Typotheriidae. Typotherium.

• Suborder (2). Condylarthra.
• Family Phenacodontidae. Phenacodus.
• Suborder (3). Hyracoidea.
• Family Hyracidae. Procavia (Hyrax).
• Suborder (4). Amblypoda.
• Family Coryphodontidae. Coryphodon.
• Family Uintatheriidae. Uintatherium (Dinoceras).
• Suborder (5). Proboscidea.
• Family Dinotheriidae. Dinotherium.
• Family Elephantidae. Mastodon.
• Elephas—elephant.
• Group Tillodontia.
• Order 5. Rodentia.
• Suborder (1). Simplicidentata.
• Section Sciuromorpha.
• Family Castoridae. Castor—beaver.
• Section Myomorpha.
• Family Lophiomyidae. Lophiomys.
• Family Muridae. Hydromys.
• Acanthomys—spiny mouse.
• Mus—mouse.
• Family Spalacidae. Bathyergus.
• Family Dipodidae. Dipus—jerboa.
• Pedetes—Cape jumping-hare.
• Section Hystricomorpha.
• Family Hystricidae. Hystrix—porcupine.
• Family Chinchillidae. Chinchilla.
• Lagostomus—viscacha.

• Family Dasyproctidae. Coelogenys—paca.
• Dasyprocta—agouti.
• Family Caviidae. Cavia—guinea-pig.
• Hydrochaerus—capybara.
• Suborder (2). Duplicidentata.
• Family Leporidae. Lepus—hare and rabbit.
• Order 6. Carnivora.
• Suborder (1). Creodonta.
• Family Hyaenodontidae. Hyaenodon.
• Suborder (2). Carnivora vera or Fissipedia.
• Section Æluroidea.
• Family Felidae. Felis—cat, lion, tiger.
• Machaerodus—sabre-toothed lion.
• Family Viverridae. Viverra—civet.
• Paradoxurus—palm civet.
• Family Protelidae. Proteles—aard wolf.
• Family Hyaenidae. Hyaena.
• Section Cynoidea.
• Family Canidae. Canis—dog, wolf, fox.
• Section Arctoidea.
• Family Ursidae. Ursus—bear.
• Family Mustelidae. Latax—sea otter.
• Suborder (3). Pinnipedia.
• Family Otariidae. Otaria—sea lion.
• Family Trichechidae. Trichechus—walrus.
• Family Phocidae. Ogmorhinus—sea leopard.
• Order 7. Insectivora.
• Suborder (1). Dermoptera.
• Family Galeopithecidae. Galeopithecus—'flying lemur'.

• Suborder (2). Insectivora vera.
• Family Macroscelidae. Macroscelides—jumping shrew.
• Family Erinaceidae. Erinaceus—hedgehog.
• Gymnura.
• Family Soricidae. Sorex—shrew.
• Family Talpidae. Talpa—mole.
• Family Potamogalidae. Potamogale.
• Family Solenodontidae. Solenodon.
• Family Centetidae. Microgale.
• Centetes—tenrec.
• Family Chrysochloridae. Chrysochloris—golden mole.
• Order 8. Chiroptera.
• Suborder (1). Megachiroptera.
• Family Pteropidae. Pteropus—flying fox.
• Suborder (2). Microchiroptera.
• Family Rhinolophidae. Horseshoe bats.
• Family Phyllostomatidae. Desmodus—vampire.
• Order 9. Primates.
• Suborder (1). Lemuroidea.
• Family Tarsiidae. Tarsius—tarsier.
• Family Chiromyidae. Chiromys—aye aye.
• Suborder (2). Anthropoidea.
• Family Hapalidae. Hapale—marmoset.
• Family Cebidae. Mycetes—howling monkey.
• Ateles—spider monkey.
• Family Cercopithecidae. Cynocephalus—baboon.
• Macacus.
• Colobus.
• Family Simiidae. Hylobates—gibbon.
• Simia—orang.
• Gorilla.
• Anthropopithecus—chimpanzee.
• Family Hominidae. Homo—man.

CHAPTER III.
SKELETON OF HEMICHORDATA, UROCHORDATA, AND CEPHALOCHORDATA.

The subphylum includes three genera, Balanoglossus[19], Cephalodiscus and Rhabdopleura; and perhaps a fourth, Phoronis.

The skeletal structures found in Balanoglossus[20] are all endoskeletal. They include:

(1) The notochord. This arises as a diverticulum from the alimentary canal which grows forwards into the proboscis and extends beyond the front end of the central nervous system. It is hypoblastic in origin and arises in the same way as does the notochord of Amphioxus. Its cells become highly vacuolated and take on the typical notochordal structure[21]. The cavity of the primitive diverticulum becomes obliterated in front, but behind it opens throughout life into the alimentary canal.

(2) The axial skeletal rods. These are a pair of chitinous rods which lie ventral to the notochord and in the collar region unite to form a single mass.

(3) The branchial skeleton. The gill bars separating the gill slits from one another are strengthened by chitinous rods in a way closely similar to that in Amphioxus. But between one primary forked rod and the next there are two secondary unforked rods—not one, as in Amphioxus.

(4) The chondroid tissue. This is of mesoblastic origin and may be regarded as an imperfect sheath for the notochord.

In Cephalodiscus and Rhabdopleura as in Balanoglossus the notochord forms a small diverticulum growing forwards from the alimentary canal into the proboscis stalk.

Recent researches on Phoronis[22] show the existence in the collar region of the larva (Actinotrocha) of a paired organ, which is regarded by its discoverer as representing a double notochord.

SUBPHYLUM B. UROCHORDATA (Tunicata).

Skeletal structures of epiblastic and hypoblastic origin occur in the Urochordata. Most Tunicates are invested by a thick gelatinous test which often contains calcareous spicules, and serves as a supporting organ for the soft body. The cells of this test are mesodermal in origin.

In larval Tunicata and in adults of the group Larvacea the tail is supported by a typical notochord, which is confined to the tail. In all Tunicata except Larvacea all trace of the notochord is lost in the adult.

SUBPHYLUM C. CEPHALOCHORDATA.

This subphylum includes the well-known genus Amphioxus[23]. In Amphioxus the skeleton is very simple. It contains no trace of cartilage or bone and remains throughout life in a condition corresponding to a very early stage in Vertebrata. The skeleton of Amphioxus is partly hypoblastic, partly mesoblastic in origin.

(a) Hypoblastic skeleton.

The notochord (fig. 3, 2) is an elastic rod extending along the whole length of the body past the anterior end of the nerve cord. It lies ventral to the nerve cord, and shows no trace of segmentation. It is chiefly made up of greatly vacuolated cells containing lymph, but near the dorsal and ventral surfaces the cells are less vacuolated. The notochord is immediately surrounded by a structureless cuticular layer, the chordal sheath, and outside this comes the mesoblastic skeletogenous layer, which also surrounds the nerve cord.

The branchial skeleton. This consists of a series of chitinous elastic rods which strengthen the gill bars and are alternately forked and unforked ventrally. The forked rods are primary, and are U-shaped in section, the unforked rods are secondary, and are circular in section. All these rods are united at intervals by transverse rods.

(b) Mesoblastic skeleton.

The buccal skeleton. On each side of the mouth there is a curved bar resembling the notochord in structure. The bars are segmented, and each segment bears a smaller rod which supports a tentacle, the whole forming the buccal skeleton (fig. 3, 4).

The notochord is enclosed in a thick sheath of connective tissue continuous with a thinner sheath round the nerve cord. The sheaths of the notochord and nerve cord together form the skeletogenous layer, and prolongations of it form the myomeres or septa between the myotomes or segments of the great lateral muscles of the body.

The skeleton of each median fin consists of small cubical masses of a gelatinous substance arranged in rows (fig. 3, 1 and 7), and serving to strengthen the fins.

CHAPTER IV.
SUBPHYLUM D. VERTEBRATA.

Order I. Marsipobranchii[24].

In these animals limbs and limb girdles are always completely absent. They have no exoskeleton except horny teeth.

The endoskeleton, excluding the notochord, is entirely cartilaginous or membranous. The axial skeleton consists of a cartilaginous cranium without jaws, succeeded by a thick persistent notochord enveloped in a sheath. The notochord in living forms is unsegmented, but segmented cartilaginous neural arches are present in some cases. A complicated series of cartilaginous elements occurs in relation to the mouth, gills, and sense organs. The median fins are supported by cartilaginous pieces, the radiale. The order includes the Lampreys and Hags.

Order II. Ostracodermi[25].

The forms included in this group have long been extinct, being known only from beds of Upper Silurian and Lower Devonian age. They differ much from all other known animals. The exoskeleton is always greatly developed and includes (1) large bony plates covering the anterior region; (2) scales covering the posterior region. The plates are deeply marked by canals belonging to dermal sense organs. Jaws are unknown, and arches for the support of the appendicular skeleton are rudimentary or absent. The tail is heterocercal (see p. 60).

Suborder (1). Heterostraci.

The exoskeleton consists principally of calcifications forming dorsal and ventral shields which cover the head and abdominal region; the dorsal shield is formed of a few plates firmly united, the ventral shield of a single plate. The shields are composed of three layers, the middle layer being traversed by canals belonging to the dermal sense organs which open to the exterior by a series of pores. The tail is sometimes covered by scales. The orbits are widely separated and laterally placed. Paired appendages are absent. These curious forms are found in beds of Upper Silurian and Lower Devonian age. One of the best known genera is Pteraspis.

Suborder (2). Osteostraci.

The exoskeleton as in the Heterostraci consists of shields and scales, the shields being divisible into three layers. The anterior part of the body is covered dorsally by a single large shield which differs from those of the Heterostraci in having the inner layer ossified. The middle layer contains canals for the passage of blood vessels, but the exoskeleton shows no impressions of dermal sense organs. The posterior part of the body is covered by large quadrangular scales. Paired appendages are absent, but median dorsal and caudal fins occur supported by scales, not fin-rays. Cephalaspis, the best known of these animals, occurs in beds of Lower Devonian age.

Suborder (3). Antiarcha.

The exoskeleton is formed of bony plates, the dorsal and ventral shields each consisting of several symmetrically arranged pieces. The tail may be covered with small scales or may be naked. The head is articulated with the trunk, and its angles are drawn out into a pair of segmented paddle-like appendages, covered with dermal plates. The orbits are close together. A dorsal fin and traces of mouth parts occur in Pterichthys, but the endoskeleton is unknown. The best known forms Pterichthys[26] and Asterolepis occur in beds of Lower Devonian age.

General account of the skeleton of

Marsipobranchii.

The Marsipobranchii are worm-like animals. The living forms include two families, the Myxinoidei (Hags)—genera Myxine and Bdellostoma—and the Petromyzontidae (Lampreys).

Three species of Petromyzon are known, P. fluviatilis, P. marinus and P. planeri. The larval forms were for a long time thought to belong to a separate genus and were called Ammocoetes.

The Myxinoids, although very highly specialised in their own way, are at distinctly a lower stage of development than the adult Lamprey, and come nearer to the larval Lamprey or Ammocoete.

Spinal column.

In Myxinoids and larval lampreys, the notochord is enclosed in a thick chordal sheath, in connection with which in the tail region there occur cartilaginous pieces forming neural arch elements. In the trunk region, however, no cartilage occurs in connection with the spinal column, the only cartilage present being that forming the radiale of the dorsal fin. On the other hand in most species of lamprey (Petromyzon) cartilaginous pieces forming imperfect neural arches (fig. 4, B, 13) are found lying in the tough skeletogenous layer dorsal to the notochord, and extending throughout the whole length of the trunk and tail. Two of these pieces, which are probably homologous with the neural plates (see p. 72) of Elasmobranchs, occur to each neuromere, or segment as determined by the spinal nerves. The dorsal and caudal fins are supported by paired cartilaginous radiale which are connected proximally with the skeletogenous layer.

The Skull.

In Myxinoids the cranium is a mere cartilaginous floor without side walls or roof, and the trabeculae[27] end without growing forwards into cornua. In Lampreys the trabeculae grow forwards and send up plates of cartilage which meet above (fig. 4, 7) and form side walls and a roof for part of the brain case. In Lampreys a labial suctorial apparatus is well developed, including a large ring-like piece of cartilage (fig. 4, 2) which supports the oral funnel and bears a large armament of horny teeth. In Myxinoids on the other hand the labial skeleton is small and consists merely of barbels round the mouth.

The olfactory organ of Myxinoids has a very curious skeleton. It is covered with a kind of grating of cartilage which is prolonged in front into a tube composed of a series of imperfect cartilaginous rings. In Lampreys the olfactory organ opens merely by a short membranous passage. In correlation with the small development of the labial suctorial apparatus in Myxinoids the lingual apparatus is very greatly developed. The tongue in Myxine has been said to 'dominate the whole body' (Parker). It is supported by a great median cartilaginous bar which when followed forwards first becomes bifid and still further forwards becomes four-cleft.

The horny teeth in Myxinoids are chiefly borne on the very large supralingual apparatus. They form a double series arranged in the form of an arch. In Myxine there are seven large teeth and nine small ones on each side. In Bdellostoma the teeth of the two rows are more equal in size. In Bdellostoma and Myxine it has been shown that imperfect calcified teeth occur below the horny teeth.

In Lampreys the lingual apparatus (fig. 4, C, 9) is well developed, but not excessively so. It consists of a long median cartilaginous bar which ends in front with a semicircular piece of cartilage supporting the median part of the tongue.

In both Myxinoids and Lampreys there is a complicated branchial basket apparatus, but while in Myxinoids the basket apparatus is interbranchial, formed deep within the head near the hypoblastic lining of the throat, in Lampreys it is extra-branchial and formed outside the head cavities (fig. 4, 10). The two sides of the basket apparatus in Myxine are not symmetrical. In the interbranchial basket apparatus of Myxinoids the hyoid and first and second branchial arches can be recognised. Traces of the interbranchial skeleton of Myxinoids can be detected in Lampreys, and similarly in Myxinoids, there are indications of the extra-branchial skeleton of Petromyzon. The branchial basket in Lampreys forms at its posterior end a kind of cup which supports the pericardium (fig. 4, 11).

A remarkable Cyclostome named Palaeospondylus[28] has recently been described from the Scottish Old Red Sandstone. It differs however from all living Cyclostomes, in having a spinal column formed of distinct vertebrae with well-developed neural arches. The caudal fin is well developed and the dorsal radiale are forked as in lampreys. The skull is well calcified and the auditory capsules are specially large. The mouth is very similar to that of lampreys, being circular and without jaws; it is provided with barbels or cirri. There is no trace of limbs and the average length is only about 1-1½ inches.

CHAPTER V.
(II.) GNATHOSTOMATA.

The centra of the vertebrae have no epiphyses. The skull may be (a) incomplete and membranous, or (b) more or less cartilaginous, or (c) bony. Membrane bones are not included in the cranial walls, and there are large unossified tracts in the skull. When membrane bones are developed in connection with the skull, a large parasphenoid occurs. The basisphenoid is always small or absent. The skull may be immovably fixed to the vertebral column, or may articulate with it by a single or double occipital condyle. When the occipital condyle is double, it is formed by the exoccipitals, and the basi-occipital is small or unossified. The mandible may be (a) cartilaginous, (b) partially ossified, or (c) membrane bones may be developed in connection with it,—if so, there is usually more than one membrane bone developed in connection with each half.

There are at least four pairs of branchial arches present during development. The sternum, if present, is not costal in origin.

Class I. Pisces.

The exoskeleton is in the form of scales, which may be entirely mesoblastic or dermal in origin (e.g. cycloid and ctenoid scales), or may be formed of both mesoblast and epiblast (e.g. placoid and ganoid scales). Large bony plates may be derived from both these types of scale. In general fish with a greatly developed dermal armour have the endoskeleton poorly developed; and the converse also holds good.

The integument of the dorsal and ventral surfaces is commonly prolonged into longitudinal unpaired fins, supported by an internal skeleton. These fins are distinguished according to their position as dorsal, caudal and anal fins. The dorsal and anal fins are used chiefly as directing organs, the caudal fin is however a most important organ of propulsion.

Three types of tail are found in fishes, viz.:—

1. The diphycercal, in which the axis is straight and the tail is one-bladed and symmetrical, an equal proportion of radiale[29] being attached to the upper and lower surfaces of the axis.

2. The heterocercal, in which the tail is asymmetrical and the axis is bent upwards, the proportion of radiale or of fin-rays attached to its upper surface being much smaller than that attached to its lower surface.

3. The homocercal, in which the tail though externally symmetrical, so far resembling the diphycercal type, is internally really heterocercal, the great majority of the radiale or of the fin-rays being attached to the lower surface of the axis.

The cranium in the simplest cases (e.g. Selachii) forms a cartilaginous box enclosing the brain and sense organs; in bony fishes it is greatly complicated. When palatine or pterygoid bones are present they are formed by the ossification of cartilage; in Sauropsida and Mammalia they are laid down as membrane bones. There is no tympanic cavity or auditory ossicle in relation to the ear.

There are two principal types of suspensorium by means of which the jaws are attached to the cranium:—

(1) The Autostylic. This is the primitive condition in which the mandibular arch articulates with the base of the cranium in front of the hyoid and in a similar manner.

(2) The Hyostylic. In this case the mandibular arch becomes connected with the hyomandibular and supported by the hyoid arch. These terms are more fully discussed in Chapter VIII.

There is always an internal framework supporting the gills; it usually consists of the hyoid arch and five, rarely six or seven, pairs of branchial arches. The limbs are represented by two pairs of fins, the pectoral and the pelvic; they are not divided into proximal, middle and distal portions. The ribs do not unite with a median ventral sternum, or meet in the mid-ventral line in any other way in the trunk region.

Order I. Elasmobranchii.

The exoskeleton is in the form of placoid scales which are sometimes so numerous as to give the whole skin a rough surface forming shagreen. In some cases the placoid scales are enlarged to form plates or spines capped or coated with enamel. These spines may be imbedded in the flesh in front of the paired or unpaired fins, or may be attached to the tail. They are specially characteristic of the suborder Acanthodii. The endoskeleton is cartilaginous and true bone is never found. Much of the skeleton, especially of the vertebral column, is however often calcified, this being especially well seen in the anterior part of the vertebral column of Rays (Raiidae). In living forms cartilaginous biconcave vertebrae are always well developed, but in some extinct forms the notochord persists unconstricted. Neural and haemal arches are however always developed; they sometimes remain separate, sometimes fuse with the centra. Ribs are often wanting and when present are often not separated off from the vertebrae. The cranium is a simple cartilaginous box whose most prominent parts are the capsules which enclose the sense organs. The skull is sometimes immovably fixed to the vertebral column, sometimes articulates with it by means of two condyles. There is no operculum and no representative of the maxilla or premaxillae. The teeth are very variable. Large pectoral and pelvic fins always occur.

The Elasmobranchii may be divided into four suborders:—

(1) Ichthyotomi.

(2) Pleuropterygii.

(3) Selachii.

(4) Acanthodii.

Suborder (1). Ichthyotomi[30].

The members of this suborder range from the Devonian to the Permian and so have long been extinct.

The endoskeletal cartilage has granular calcifications evenly distributed throughout it. The notochord is unconstricted, but the neural and haemal arches are well-developed, and the neural spines are long and slender. There is a continuous dorsal fin with separate basalia and radiale. The tail is diphycercal, and the pectoral fins are typical archipterygia[31]. The pelvic fins of the male are prolonged to form claspers.

The best known of these primitive Elasmobranchs are the Pleuracanthidae.

Suborder (2). Pleuropterygii.

This suborder was formed for the reception of Cladoselache, an Elasmobranch found in the Lower Carboniferous of Ohio[32].

The exoskeleton is in the form of small, thickly-studded dermal denticles. The vertebral centra are unossified, and the tail is strongly heterocercal. There were certainly five, perhaps seven gill slits, and the suspensorium is apparently hyostylic. The paired fins are, according to the view which derives them by concentration from continuous lateral folds, the most primitive known (see p. 129) and claspers are absent.

Suborder (3). Selachii.

Cartilaginous or partially calcified biconcave vertebrae are always well developed; they constrict the notochord intervertebrally. The neural and haemal arches and spines are stout and intercalary cartilages (interdorsalia) are present. The tail is heterocercal, but in some cases (Squatina) approaches the diphycercal condition. In most cases the suspensorium is hyostylic, the jaws being attached to the cranium by means of the hyomandibular, and the palato-pterygo-quadrate bar not being fused to the cranium. There are generally five pairs of branchial arches, and gill rays are borne on the posterior surface of the hyoid arch, and on both the anterior and posterior surfaces of the first four branchial arches. The Notidanidae differ from most Selachians in two respects, first as regards the suspensorium,—Meckel's cartilage articulating directly with the palato-pterygo-quadrate bar, and not being connected with the hyoid arch; and secondly as regards the number of branchial arches,—six pairs occurring in Hexanchus and seven in Heptanchus.

The pectoral fins are without the segmented axis of the archipterygium. In most cases they are sharply marked off from the body and lie almost at right angles to it; but in the Rays they have the form of lateral expansions in the same plane as the body, from which they are not sharply marked off. The pelvic fins in the male bear long grooved cartilaginous rods which are accessory copulatory organs or claspers.

There are two principal groups of Selachii, the Squalidae or Sharks and Dogfish, and the Batoidei or Skates and Rays. The Squalidae have the shape of ordinary fish, the pectoral fins are vertically placed and the body ends in a powerful heterocercal tail. The Batoidei have flattened bodies owing to the great size and horizontal position of the pectoral fins. The tail is long and thin and is often armed with spines. The teeth in Selachii differ much in character in the different forms, and are always arranged in numerous rows. They are generally pointed and triangular or conical in the Squalidae, while in the Batoidei they are often broad and flattened.

Suborder (4). Acanthodii.

The fishes included in this group are all extinct and in some respects are intermediate between Elasmobranchii and Ganoidei. The body is elongated and closely covered with small scales consisting of dentine enamelled at the surface. The notochord is persistent and the calcification of the endoskeletal cartilage is only superficial. The tail is heterocercal. The jaws bear small conical teeth, or in some cases are toothless. The skeleton of all the fins differs from that of modern Elasmobranchs in having the cartilaginous radiale much reduced, and the fins are nearly always each provided with an anterior spine, which except in the case of the pectoral fins is merely inserted between the muscles. These spines are really enormous dermal fin-rays; the pectoral fin-spine is articulated to the pectoral girdle.

The suborder includes many well-known extinct forms like Acanthodes and Diplacanthus; it ranges from the Devonian to the Permian.

PISCES, HOLOCEPHALI.

Order II. Holocephali.

This order includes a single suborder only.

Suborder. Chimaeroidei.