The parietes of the teeth are deeply plaited and folded, so

as to give rise to a complicated 'labyrinthine' pattern in the transverse section of the tooth."

The points in which the Labyrinthodonts differ from the modern Urodela are chiefly to be found in the fact that the head is defended by an external covering or helmet of hard and polished osseous plates, in the possession of ventral integumentary scutes, in the existence of exoskeletal plates occupying the place of the interclavicle and clavicles, in the amphicœlous form

Fig 309-Foot-prints of a Labyrinthodont (Cheirotherium), from the Trias. upper figure shows a single foot-print enlarged; the lower figure shows a slab, with several prints, and traversed by reticulated desiccation-cracks.

The

of the dorsal vertebræ, and in the complicated structure of the teeth. These last-mentioned organs are not only often very numerous, but are of large size. The subjoined illustration (fig. 310) shows the beautiful and complex structure of the teeth, from which the name of the order is derived.

Fig. 310.-Section of the tooth of Labyrinthodon (Mastodonsaurus) Jageri.

The Labyrinthodonts range from the Carboniferous Rocks to the Trias; but some of the forms commonly included in this order may perhaps belong elsewhere. One type of the Labyrinthodonts is constituted by the singular genus Archegosaurus, and the less known Apateon-both from the Carboniferous Rocks. Archegosaurus is remarkable in having the notochord persistent, and in the possession of permanent branchial arches. It has been made by Professor Owen the type of a separate group, the Ganocephala; but it is probably an immature and larval form. The occipital condyles, also, do not seem to have been ossified in the Archegosauria.

Of the Carboniferous Labyrinthodonts the most important genera are Anthracosaurus, Pholidogaster, Ophiderpeton, Ichthyer peton, Urocondylus, Lepterpeton, Baphetes, Raniceps, Dendrerpeton, Hylerpeton, and Hylonomus; though the affinities of some of these are more or less doubtful. Most of the Carboniferous Labyrinthodonts were of comparatively small size; but some, such as Baphetes (fig. 311) and Anthracosaurus, must have attained gigantic dimensions. All the above-mentioned genera seem to have possessed well-ossified vertebræ, with welldeveloped limbs, the form of the body being mostly salamandriform, but sometimes fish-like, or serpentiform.

In the Permian Rocks, a few remains of Labyrinthodonts have been discovered, the genus Zygosaurus being peculiar to this period.

Fig. 311.-Baphetes planiceps, from the Carboniferous Rocks of Nova Scotia. (After Dawson.) a Anterior part of the skull, viewed from beneath, and much reduced; ¿ Oneof the largest teeth, natural size.

In the Triassic Rocks the remains of Labyrinthodonts are abundant, the most important genus being Labyrinthodon or Mastodonsaurus. This genus is known mainly by footprints and by crania; and the size attained by some species must have been colossal. No remains of this order have hitherto been discovered in rocks younger than the Trias.

CHAPTER XXXII.

REPTILIA.

THE true Reptiles and the Birds, unlike as they are in external appearance, are nevertheless related to one another by various points of affinity; so that they may well be included in a single division, which has been termed Sauropsida by Huxley. The Sauropsida are defined by the possession of the following characters:-At no period of existence are branchiæ, or waterbreathing respiratory organs, developed upon the visceral arches; the red corpuscles of the blood are nucleated; the skull articulates with the vertebral column by means of a single articulating surface or condyle; and each half or "ramus" of the lower jaw is composed of several pieces, and articulates with the skull, not directly, but by the intervention of a peculiar bone, called the “quadrate bone," or "os quadratum" (fig. 312).

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These being the common characters of Reptiles and Birds, by which they are collectively distinguished from other Vertebrates, it remains to inquire what are the characters by which they are distinguished from one another. The following, then, are the characters which separate the Reptiles from the Birds: -The blood in Reptiles is cold—that is to say, slightly warmer than the external medium-owing mainly to the fact that the pulmonary and systemic circulations are always directly connected together, either within the heart or in its immediate neighbourhood, so that the body is supplied with a mixture of venous and arterial blood, in place of pure arterial blood alone. The terminations of the bronchi at the surface of the lung are closed, and do not communicate with air-sacs, placed in different parts of the body. When the epidermis develops horny structures, these are in the form of horny plates or scales, and never in the form of feathers. The fore-limbs are formed for various purposes, including in some cases even flight, but they are never constructed upon the type of the "wings" of Birds. Lastly, with one or two doubtful exceptions, whilst the ankle-joint is placed between the distal and proximal portions of the tarsus, the tarsal and metatarsal bones of the hind-limb are never anchylosed into a single bone.

These are the leading characters by which Reptiles are distinguished from Birds; but we must not forget the other distinctive peculiarities in which Reptiles agree with Birds, and differ from other Vertebrates-namely, the absence of branchiæ at all times of life, the possession of only one occipital condyle, and the articulation of the complex lower jaw with the skull by means of a quadrate bone.

It is now necessary to consider these characteristics of the Reptilia a little more minutely. The class includes the Tortoises and Turtles, the Snakes, the Lizards, the Crocodiles, and a number of extinct forms; and with the exception of the Tortoises and Turtles they are mostly of an elongated cylindrical shape, provided posteriorly with a long tail. The limbs may be altogether absent, as in the Snakes, or quite rudimentary, as in some of the Lizards, but as a general rule both pairs of limbs are present, sometimes in the form of ambulatory legs, sometimes as swimming-paddles, and in some extinct forms modified to subserve an aerial life. The endoskeleton is always well ossified, and is never cartilaginous or semi-cartil-· aginous, as in many Fishes and some Amphibians. The skull articulates with the atlas by a single condyle. The lower jaw is complex, each half or ramus being composed of from four to six pieces, united to one another by sutures (fig. 312). In the Tortoises, however, these are anchylosed into a single piece,

and the two rami are also anchylosed. In most Reptiles, however, the two rami of the lower jaw are only loosely unitedin the Snakes by ligaments and muscles only, in the Lizards by fibro-cartilage, and in the Crocodilia by a regular suture. In all, the lower jaw articulates with the skull by a quadrate bone (fig. 312, a); and as this often projects backwards, the

Fig. 312.-Skull of a Serpent (Python). b Articular portion of the lower jaw; a Quadrate bone; c Squamosal portion of the temporal bone.

opening of the mouth is often very extensive, and may even extend beyond the base of the skull. Teeth are usually present, but are not sunk in separate sockets or alveoli, except in the Crocodiles and in some extinct forms. In the Tortoises and Turtles alone of living types there are no teeth, and the jaws are simply sheathed in horn, constituting a kind of beak like that of a bird.

Ribs are always present and always well developed, but they differ much in form. It is not correct, however, to regard the presence of ribs as separating the true Reptiles from the Amphibia, as is sometimes stated. Some of the most Lizard-like of the Amphibians, such as the Siren, possess short but welldeveloped ribs, and rudiments of ribs are traceable in other orders; whilst in the Cæcilia they are large and well developed. As regards the exoskeleton, all Reptiles have horny epidermic scales, and they are divided into two great sections-called respectively Squamata and Loricata-according as the integumentary skeleton consists simply of these scales, or there are osseous plates developed in the derma as well. In the Tortoises, the epidermic plates unite with the bony exoskeleton and with the true endoskeleton to form the case or box in which the body of these animals is enclosed.

The class Reptilia is divided into the following nine orders, of which the first four are represented by living forms, whilst the remaining five are extinct: