REPTILES. 113 



fifth costal arches, are very delicate measurements of the degree of metamorphosis of these parts : 

 all these parts, namely, Sternum and sternal ribs, are ossified endosteally, and the Sternum itself 

 by symmetrical halves (see fig. 2). Plate X, fig. 6, shows a section through the lower part of the 

 coracoid magnified ten diameters; the double layer of endosteal bone (en. o.) in the epicoracoid 

 (e. cr.) is well shown. 



Example 4. Trachydosaurus rugosus, Gray. 



This great Australian Lizard is perhaps still more instructive than its relative just described. 

 In this species the supra-scapula (Plate X, fig. 3, s. sc.) is smaller in proportion to the scapular 

 shaft (sc.) than in the last ; and this latter bone is but little cleft above : the cleft, however, is 

 evident, and shows what a fenestra is at its first appearance. The broad, badly defined meso- 

 scapula (acromion) is bordered by a narrow band of soft cartilage (fig. 3, p. sc.), so that there is 

 here less continuity of the internal bone than in Cyclodus ; also the crest of the supra-scapula is 

 soft (fig. 3). The coraco-scapular notch (fig. 3) is smaller, and therefore we have here a very wide 

 neck to the scapula, and the oblique suture connecting it with the coracoid is very long. This 

 latter bone (cr.) is very broad, and is only bifurcate, so that in this case also there is only one cora- 

 coid fenestra; this a long ellipse (see fig. 5, c. f.). The epicoracoids (fig. 5, e. cr.) are nearly as 

 large, relatively, as those of a Salamander ; and, as in that creature, there seems to be some varia- 

 tion in the overlapping of these flaps, for as fig. 5 shows the left overlies the right; contrary 

 to what I have shown in the Iguana (Plate IX, fig. 2). 



The homologies of the Shoulder-splints in Trachydosaurus appear to me to be self-evident ; 

 for this creature has positively retained the very self-same kind of clavicles as those possessed 

 by the ordinary Osseous Fish. Were the supra-scapula absent, as in the Teleostei, and the rest 

 of the Shoulder-girdle moiety relatively somewhat smaller, then surely even a transcendentalist 

 would acknowledge the unity of the clavicles here figured (Plate X, figs. 3 5, cl.), and those 

 of an ordinary Bony Fish. In fig. 3 it is shown that the clavicle passes above the scapula, and 

 that it broadens out below into a large convexo-concave plate, which is applied, like a paste- 

 board splint, to the front and outside of the Shoulder-plate. Fig. 5 shows that these clavicles 

 have their thick front edges turned over the front of the prse-coracoid, and that the endo- 

 skeletal parts lie folded in and embraced by these scooped exo-skeletal bones, exactly as in 

 the Fish. 



Fig. 4, displaying these bones from below, shows the not infrequent membranous space 

 (see Plate IX, fig. 8, cl., and Plate XIII, figs. 1 and 2, cl.), and also the wild, jagged, Fish-like 

 hinder margin of the bone. If, however, this bone is illustrated by what is seen in the lower 

 Class, it also typifies still higher forms, and foreshadows what is seen in the embryo of the Bird. 

 The relation of the clavicles to the inter-clavicle (fig. 4, cl., i. cl.) is perfectly Reptilian, and they 

 are seen to correspond completely to the three foremost bones of the Chelonian plastron (see 

 Plate XII). The inter-clavicle (fig. 4, i. cl.) has its four rays more nearly equal than in Cyclodus; 

 the lateral rays, a little the smallest, are both decurved and turned upwards, and all the rays are 

 less spatulate than in the last example : the hindermost underlies the prse-sternal notch (see figs. 

 4 and 5). 



The antero-lateral margins of the Sternum (fig. 4) are longer, by far, than the postero- 

 lateral ; this arises, from the large room required by the huge epicoracoids ; the left lower coracoid 

 ' 15 



