Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/471

Rh SKELETON.] REPTILES 453 broad, more or less cartilaginous, or variously ossified median portion. THE APPENDICTTLAR SKELETON. The appendicular skeleton of Reptiles, like that of Vertebrates generally, FIG. 20. Lateral aspect of skull of Clielys matamata (from nature), an, angular ; ar, articular; bo, basioccipital ; d, dentary; ep, epiotic ; m, maxilla; pa, parietal ; pin, premaxilla ; pi; prefrontal ; ps, postfrontal ; ft, pterygoid ; q, quadrate ; s, squamosal ; sg, surangular. consists of a pair of limb girdles (pectoral and pelvic), with a pair of skeletal appendages proceeding from either girdle. Each such skeletal appendage has also, normally, the typical differentiation into (1) a single upper limb bone, (2) a pair of lower limb bones, (3) a group of small foot-root bones or cartilages (carpus or tarsus), (4) a series of middle foot bones (metacarpus or metatarsus), and (5) pm mdl Fio. 21. Ventral aspect of skull of Clielys matamata (from nature), bo, basioccipital ; bs, basisphenoid ; mdl, mandible; oh, opisthotic ; pi, palatine; pm, premaxilla ; po, prootic ; pt, pterygoid ; q, quadrate ; s, squamosal. a group of small bones (phalanges) arranged in series according to the number of digits, but the number in each digit varying in different digits and in different groups of Reptiles. Every trace of an appendicular skeleton may, however, be wanting, as is the case in most Ophidians. There may be a pectoral limb girdle without any rudi- ment of a fore-limb skeleton, as in Anguis, Acontias, and Amphisbsena. There may be a pelvic girdle without any rudiment of a hind-limb skeleton, as in at least some, if not all, Typhlopidx,. Very rarely there may be both rudimentary pectoral and pelvic girdles without any trace of limb skeleton, as in Amphisbxna. Thus, of the two categories, (1) limb girdles, and (2) girdle appendages or limbs, the former is the more constant, as we never find rudimentary extremities and limbs without any trace of a girdle, while the contrary we do find here and there. 1 The Limb Girdles. Each girdle consists of two lateral halves, and each such lateral half further consists of two 1 This is interesting because it is probable that genetically the limb i ; prior, to the limb giivlle, which v.'as an ingrowth from the former. See Trans. '/, 1. Soc., 1879, vol. x. p. 439. divisions which diverge from the articular surface offered to the limb by such lateral half. From that articular surface one division descends dorsally and does not con- nect itself with its fellow of the opposite side. The other ventral division passes inwards and somewhat downwards and normally does connect itself, directly or indirectly, with its fellow of the opposite side. The dorsal ascending division is generally single, and consists of two parts in serial succession. The ventral division consists normally of two or more parts, which, though diverging, pass ven- trally side by side, or one in front of the other. Each girdle normally connects itself with the axial skeleton either towards the dorsal or ventral aspect of the latter. The Pectoral Girdle. This girdle only becomes con- Pector nected (except in Chelonians) with the ventral part of the g irdle - appendicular skeleton, to wit, where it abuts against either side of the sternum. In its most complex condition that found in some Saurians, e.g., Iguana the dorsal division consists of a scapula (with a forwardly extending FIG. 22. Sternum and shoulder girdle of AmUyrhynchus tubcristatus (after Stein, dachner). cl, clavicle; co, coracoid ; h, humerus; ic, intcrclavicle ; me, meso- coracoid ; ms, mesoscapula; pc, prccoracoid; s, scapula; tt, sternum. process the mesoscapular), and is continued upwards into a suprascapular ; while the ventral division consists of a coracoid, which abuts against the sternum, and from which ^ three parts (the precoracoid, o t the epicoracoid, and the meso- FlG 2 3.-Rudiments of pectoral arch- COraCoid ) diverge forwards I, of Acontias meJeagns ; 2, of Typhlo- ,, , . ,. saurtis aurantiaeus(aiterliiibinger). and inwards, and also of a clavicle, between which and its fellow of the opposite side an interclavicle is interposed. This girdle normally in Reptiles, as in all other Vertebrates, embraces more or less of the axial skeleton externally ; only in adult Chelonians it becomes drawn in within the circuit of the ribs. In the Ophidia, as has been said, there is no pectoral girdle. In certain Sauria it is in its simplest possible form, as, e.g., in Acontias and Typhlosaurus, where each lateral half consists of a minute ossification representing the clavicle and coracoid, which is not connected with its fellow of the opposite side by any hard structure, as both sternum and interclavicle are wanting. It is also quite rudimentary in Amphisbxna, but it is fairly developed in Chirotes. Its most complex condition in Saurians has been already described. The Chamscleons have a simple girdle consist- ing of a long and narrow scapula with suprascapula and a' long and narrow coracoid, the anterior part of which