Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/472

Rh 454 REPTILES [ANATOMY. envelops (or is overlapped by) its fellow of the opposite aide, while its hinder part joins the sternum. There is no clavicle or interclavicle. Hatteria has both a scapula with a mesoscapular pro- cess and a cartilaginous suprascapula, also a coracoid with a large cartilaginous epicoracoid. There is an inter- clavicle, the diverging anterior arms of which, as clavicles, are connected by ligament with the scapula. The Crocodilia have a simple girdle, consisting only of a scapula with a cartilaginous suprascapula and a cora- coid without processes or epicoracoid, nor are there any clavicles but only a median anteroposteriorly directed interclavicle. In the Chelonia the girdle lies between the dorsal and ventral shields. The dorsal division is a columnar scapula, which ascends to be connected by ligament or cartilage, which sometimes contains points of ossification, with the transverse process of the first thoracic vertebra. Of the bifold ventral division, the anterior bone is a direct con- tinuation of the scapula, and connects itself with the inter- clavicle of the plastron, whilst the posterior bone is a coracoid and ends freely (there being no sternum). In Sphargis the end of the right coracoid underlies that of the left coracoid. TJie Pelvic Girdle. This girdle only becomes connected with the dorsal part of the appendicular skeleton to wit, where it abuts against the sacral vertebra?. In general each of its halves consists of a dorsal divi- sion more or less simple and columnar, the ilium, united to the vertebral column by cartilage, and of a bifold ventral division, the parts of which, pubis and ischium, unite in the middle line by the intervention of a median longi- tudinal cartilage or ligament, a process extending forwards from the front margin of the more anterior part the pubis while an azygous bone, the os cloacae, extends back- wards from behind the symphysis formed by the junction of the more posterior part, the ischium, with its fellow of the opposite side. The ilium, pubis, and ischium form the acetabulum. Although it has been elsewhere already described, it may be well again to mention here that in certain ex- tinct Reptiles not only is the ilium greatly expanded above antero-posteriorly but the pubis sends back a long slender post-pubis nearly parallel with the ischium and closely resembling the so-called pubis of Birds. 1 In the Ophidin the pelvic girdle is generally entirely absent, and, when a rudiment is present, that rudiment is never united with the axial skeleton. It may consist merely of a pair of subparallel slender pubic spicules or cartilages, as in Typldops, or of a rudimentary ilium, pubis, and ischium on each side, as in Cylindropkis, Ilyaia, Stenostoma, and Bon. A rudimentary pelvis also exists in Python, Eryx, and Tortrix. In Saurians the girdle may also be detached from the axial skeleton and most rudimentary, as in Lepidostemon, where it consists of no more than in Typh- lops, and in Anguis, where each half of it consists of an elon- gated ossicle with three processes Correspond- FIG- 24. Rudiments of pelvic limb 1. of Lialit , *~ 4-1,- it,, Bartonii; 2, of Anguis fragilis; 3, of Amphis- ing to the three nor- blKna f u ng ittosa. ^femwTw, ffium; i>, nio- mal pelvic bones. It Perineum; P, pubis; t, tibia. may be even more simple and yet attached to the verte- bral column, as in Acontias, Pseudopus, and Ophisaurus. 1 See especially a paper by Professor Marsh in Amer. Jour, of Sci. and Arts, vol. xvii. (January 1879), pi. 8, and one by Dollo in the Bull. Mus. Roy. d'Hist. Lot. de Belgirjue, vol. ii., 1883, " Troisieme note sur les Dinosaurs, pi. 3.

J In all Saurians with ordinary hind limbs the pelvic girdle is fully developed. The Chelonians have long ilia nearly vertical in posi- tion and with a cartila- ginous margin. The pubis and ischium form an acute angle, and are only connected with their fellows of the opposite side by liga- ment. There is no os cloacae. Hatteria has a la- certilian pelvis. The FIG. pubis has a small for- wardly extending pro- 25. 1 1, Rudimentary pelvis and limb of Stenoitoma niacrolepis. 2, The same parts of Boa (after Furbinger). /, femur; iV, ilium ; ip, bone 'culled " iliopectineum " by Fiirbinger; p, pubis ; /, tibia. cess, and the ischium a much more prominent backwardly extending one. In the Crocodilia the acetabulum is ^formed by the ilium and ischium only, and is imperfectly ossified inter- nally. The two ischia are united by synchondrosis, and each develops a forwardly extending process contributing to close the acetabulum, from which the pubic bone extends forwards, downwards, and inwards. The two pubic bones are only united together by membrane. The Chelonia have the pelvic girdle, like the pectoral one, enclosed between the carapace and plastron. In most forms it is not united with either shield, the ilium being merely attached to the transverse processes of the sacral vertebrae. In some forms, however, as in Chelys, Chelodina, Pelomedusa, and others, the ilia firmly unite Avith the posterior plates of the carapace, while the ischia and pubes unite with the plastron. The three bones concur to form the acetabulum, and the two pubes and the two ischia respectively unite in ventral symphyses. The space be- tween the pubis and ischium of either side, the obturator foramen, is in Chelone and Sphargis separated from that of the other side by a ligament only, which proceeds from the ischiatic symphysis forwards to the pubic symphysis. In the Land Tortoises, however, the pubes and ischia so expand .ventrally that the obturator foramen is (on each side) enclosed by the junction of the pubis and ischium of either side at their distal ends. Each pubis has ordinarily a downwardly bent spinous process at its anterior margin, and the ischium often has a process projecting from its hinder margin. The pubes are generally widely expanded bones, very much larger than either the ischia or the ilia. The Pectoral Limb. The skeleton of this limb in its full Pec and normal differentiation resembles in its main features lini1 that of Mammals; it may, however, be simplified to a greater extent than in any other air-breathing Vertebrate class, and this according to two modes of simplification. Thus it may be a relatively minute member ending in what is (at least practically) but a single digit, as in Rhodona? It may, on the contrary, be simplified by being made up of parts all so exceedingly similar that the typical differentiation of the limb can be with difficulty traced, while the digital elements seem to indicate more than pentadactylism, as in the extinct Ichthyosauri. A peculiar complexity and extreme differentiation of this limb, however, exists, as is well known, in another extinct group, the Pterodactyles, wherein the outermost or ulnar digit is enormously elongated so as to support the wing membrane as do the four digits of existing Bats. The following are the main conditions of the limb met with in the existing Reptilian orders. In most Lacertili'i there is an elongated humerus, the proximal end of which is compressed and furnished with two tuberosities. Of the 2 See Gray, Ann. Nat. Hist., ii. 335 ; and Giinther, Ann. end Mag. Nat. Hist., 1867, xx. p. 46.