REPTILES. 10? 



In regard to the development of this unsymmetrical appendage to the Sternum of some of the 

 Scaly Reptiles, we may presume that in these instances a symmetrical portion of the Sternum, the 

 moieties of which in these Scaly Reptiles remain separate and form two moderately or considerahly 

 prolonged horns, here in an exceptional manner lie close together, and ultimately coalesce. That this 

 is the mode of development is indicated alike by the spaces in Chamasaura anguina and the two short 

 horns in Ameiva vulgaris, and also the circumstance that in one of three specimens of Cyclodus 

 nigroluteus [see the figures of this and of Trachydosaurus, Plate X, figs. 2 and 4], I found, as usual, 

 the horns of the Sternum were of moderate length, and divided iu their whole extent ; in a second they 

 had completely coalesced at their extremities, and in a third for a moderately long extent in the middle. 



These last remarks of Rathke show how profound his conceptions were on these subjects. 

 They are perfectly true to nature ; and I hope, with the help of ray figures, to illustrate these 

 important researches of his, to throw still more light upon the matter, and to give to this branch 

 of morphology an organic unity. If my descriptions are found in any case to differ from his, 

 it will often be explicable from the fact of the great variation to be seen even in the same 

 species, as his description of the Sternum of the three individuals of Cyclodus nigroluteus testifies. 

 But the description of the Sternum and of its interdavicular splint forms only part of the plan 

 of this Memoir, for I have also to give an account of the Shoulder-girdle, and of the symmetrical 

 splints (clavicles) that are strapped upon it. 



Example 1. Iguana tuber culata, Laurenti. 



In giving instances of the structure of the Shoulder of the typical Lacertilia, I need not 

 follow any very strict zoological system, but may take them as they best come in for illustration. 



The common Iguana possesses the most perfect Shoulder-plates, so that it may come first, 

 and be used as a measure for the rest, and the large amount of arrested cleavage each plate has 

 undergone makes it a most instructive example. These plates are, as it were, hacked into large 

 holes (fenestrse), but there is no perfect cleft through the large osseo-cartilaginous plate. Each 

 moiety of the Shoulder-girdle is ready to become a whole series of phalangoid rays, but there are 

 here, as in the Reptilia generally, only two perfect shaft-bones, and no part of the plate is quite 

 divided ; \ve therefore miss the supra-scapular outer plate, and also that of the prae-coracoid ; 

 these regions are mostly affected by " endostosis," and this is arrested so as to leave a selvedge 

 of clear cartilage. Plate IX, fig. 1, shows a side view of the lower part of the Iguana's thorax, 

 of the natural size; and fig. 2 represents the same as seen from" below. The supra-scapula (s. sc.) 

 is obliquely fan-shaped ; it is large, and has a five-fingered en dosteal plate within, which keeps quite 

 clear of the scapula below, does not reach the front margin, but does extend to the arcuate hinder 

 edge. There is no indication of cleaving in the supra-scapula, save the lobulate manner in which 

 the bony layers pass upwards into the broad part ; but the rest of the Shoulder- plate is perforated 

 with large open spaces, or " fenestra3." The longer axes of these oval windows are arranged 

 in a fan-like manner; the upper looking upwards and forwards, the two lower downwards and 

 forwards, whilst the top but one is directed horizontally. The lowest space is the smallest ; the 

 uppermost next ; next to that comes the top but one, and the lowest but one is the largest. 



The longer axis shows the direction of the splitting of the cartilaginous plate, which 

 fission stopped short at each end, but spread laterally. These fenestrae, from below downwards, 

 may be called the scapular (figs. 1 and 2, sc. f.), the coraco-scapular (c. s. f.), the upper coracoid 



