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GENERAL CHARACTERS] skeleton only. They attempted a further grouping of the orders which in Owen's system were merely serially enumerated

cosubordinate groups. used for this purpose almost exclusively the position and character of the rib-articulations to the vertebral centra, the orders themselves being the same as in Owen's system:—

A. PLEUROSPONDYLIA. Dorsal vertebrae devoid of transverse processes and not movable upon one another, nor are the ribs movable upon the vertebrae. A plastron. Order 1,.

B. The dorsal vertebrae (which have either complete or rudimentary transverse processes) are movable upon one another, and the ribs upon them. No plastron. a. The dorsal vertebrae have transverse processes which are either entire or very imperfectly divided into terminal facets (). . Transverse processes long; limbs well developed, paddles; sternum and sternal ribs absent or rudimentary. Order 2, ( = Sauropterygia, Ow.).

. Transverse processes short. aa. A pectoral arch and urinary bladder. Order 3, .

bb. No pectoral arch and no urinary bladder. Order 4, . b. The dorsal vertebrae have double tubercles in place of transverse processes (). Limbs paddle-shaped. Order 5, ( = Ichthyopterygia, Ow.).

c. The anterior dorsal vertebrae have elongated and divided transverse processes, the tubercular being longer than the capitular division (). . Only two vertebrae in the sacrum. Order 6, .

. More than two vertebrae in the sacrum. aa. Manus without a prolonged ulnar digit. . Hind limb Saurian. Order 7, ( = Anomodontia, Ow.).

. Hind limb Ornithic. Order 8, ( = Dinosauria, Ow.). bb. Manus with an extremely long ulnar digit. Order 9,.

, by combining the modifications of the quadrate and supporting bones with the characters used by Huxley, further

developed Owen's classification, separating the Pythonomorpha and Rhynchocephalia as distinct orders from the Lacertilia. He eventually elaborated the following classification, based entirely on osteological characters:—

I. The quadrate bone immovably fixed to the adjacent elements by suture. II. The quadrate bone loosely articulated to the cranium and at the proximal end only (Streptostylica). No distinct supramastoid, nor opisthotic; one or no postorbital bar; scapular arch, when present, external to ribs; ribs one-headed. Order 12,.

While this classification was being considered and prepared, both Cope and G. Baur made a special study of the bones which surround the quadrate and arch over the biting muscles in the various groups of reptiles. This led to a series of discussions which ended in the idea, that the class could be most naturally divided into two great subclasses, the one culminating in tortoises and mammals, the other in crocodiles, lizards, snakes

and birds. Professor H. F. in 1903 therefore proposed the following classification:—

Subclass. Primarily with single or undivided temporal arches. Giving rise to the mammals through some unknown member of the Anomodontia.

Orders Cotylosauria, Anomodontia, Testudinata and Sauropterygia.

Subclass. Primarily with double or divided temporal arches. Giving rise to the birds through some unknown type transitional between Protorosauria and Dinosauria.

Orders Diaptosauria ( = Protorosauria, Pelycosauria and Rhynchocephalia), Phytosauria ( = Belodon, &c.), Ichthyosauria, Crocodilia, Dinosauria, Squamata and Pterosauria.

The most exhaustive and modern general work on reptiles is by Dr C. K. in Bronn's Klassen und Ordnungen des

Thierreichs (1879-90). A most useful and less technical treatise is the volume on Amphibia and Reptiles contributed by Dr H. Gadow to the Cambridge Natural History (London, 1902).
 * (A. C. G.; A. S. Wo.)

Reptiles, as known in the existing world, are the modified, and in many respects degenerate, representatives of a group of lung-breathing vertebrate animals which attained its maximum development in the Mesozoic period. So far as can be judged from the skeleton, some of the members of this group then living might have become mammals by very slight change, while others might as readily have evolved into birds. It is therefore probable that the class Reptilia, as now understood, comprises the direct ancestors both of the Mammalia and Aves. Assuming that its extinct members, which are known only by skeletons, were organized essentially like its existing representatives, the class ranks higher than that of the lowest five-toed vertebrates (class Batrachia) in the investment of the foetus by two membranous envelopes (the amnion and allantois), and in the total absence of gills even in the earliest embryos. It ranks below both the Mammalia and Aves in the partial mixture of the arterial blood with the venous blood as it leaves the heart, thus causing the organism to be cold-blooded; it also differs both from Mammalia and Aves in retaining a pair of aortic arches, of which only the left remains in the former, while the right one is retained in the latter. No feature in the endoskeleton is absolutely distinctive, except possibly the degeneration of the parasphenoid bone, which separates the Reptilia from the Amphibia. In the exoskeleton, however, the epidermis forms horny scales, such as never occur in Amphibia, while there are no traces of any structures resembling either hairs or feathers, which respectively characterize Mammalia and Aves.

There is little doubt that true reptiles date back to the latter part of the Palaeozoic period, but at that epoch the Amphibia approached them so closely in the characters of the skeleton that it is difficult to distinguish the members of the two classes among the fossils. Some of the Palaeozoic Amphibia—a few of the so-called Labyrinthodonts—are proved to have had well developed gill-arches in their immature state, while there are conspicuous marks of slime-canals on their skulls. Others are