134 SHOULDER-GIRDLE AND BREAST-BONE. 



second rib of the same young Turtle is shown, magnified six diameters ; here it is seen that towards 

 the lower part of the rib, the ectosteal sheath, at this stage, has not spread into the surrounding con- 

 nective tissue, and that the rounded and flattened end is still unossified. 



Rathke's description of the Plastron extends from p. 122 to 131 : he commences with 

 Sphargis coriacea. 



RATHKE ' Ueber die Entwickelung der Schildkrdten,' pp. 122 131 : 4to, Brunswick, 1848. 



I found the Plastron most feebly developed in the young of Sphargis. It here consisted of four 

 symmetrical arched and both relatively and absolutely extremely diminutive linear structures, every- 

 where of nearly equal breadth, and so arranged that the anterior pair were at a considerable distance 

 from the posterior, and came into actual contact in the middle line, while the inner ends of the 

 posterior pair only came into close proximity with one another at that point (plate iv, fig. 5, a and b). l 

 Each strip 2 presented an osseous structure at its two extremities, whilst the middle part, for a con- 

 siderable extent, was cartilaginous [gristly, not composed of hyaline cartilage]. There were thus 

 eight bony centres for the four processes. From near the posterior extremity of each anterior strip 

 there proceeded a short, simple, pointed, horizontal process, running outwards, and representing the 

 so-called ala of the Plastron. I was unable to discover any trace of an azygous piece. In a some- 

 what older specimen of Sphargis the Plastron was almost wholly ossified ; but even here I was unable 

 to discover any indication of a median piece. The eight osseous pieces of which it consisted were, even 

 collectively, but small and thin. Those belonging to the anterior pair were the broadest, and presented 

 at their posterior border a longitudinal fissure, which became deeper towards the inner extremity of the 

 plate, making it appear at this part as if there were two bony laminae, one superimposed upon the 

 other. The osseous pieces of the anterior pair, moreover, sent forwards from their point of contact 

 two diverging processes (plate ix, fig. 2 3 ) of moderate length, very thin, but of considerable breadth, 

 and of much greater size than in the younger specimens, where, however, they could also be perceived. 

 The bony pieces constituting the second pair still lay at a considerable distance from those of the 

 third; but whether or no [he adds in a note] in adults of the species Sphargis, the several bony pieces 

 of the Plastron have only a small breadth, and if the azygous piece is absent, [he does not know] . 



In regard to the grade of development, the next to follow are the young of Chelonia virgata 

 (plate vi, fig. 22). In these the Plastron* consists of nine bony plates, which are not connected with 

 one another bv cartilage, and of which the first pair are in close apposition to the second, and those of 

 the third with the fourth. On the other hand, the second pair are farther distant from one another 

 than is the case in the Sphargis. Moreover, the two anterior and the two posterior symmetrical pieces 

 were on the whole small, whilst each of the four middle pieces had sent out, at nearly a right angle, 

 and laterally, a moderately broad and long ala, which divided at its outer extremity into two hori- 

 zontally disposed crura. The azygous piece was very diminutive, and was placed immediately behind 

 the first pair, where these came into contact by their inner or anterior extremities. The Plastron of 

 a young Chelonia imbricata presented a very similar conformation, except that its constituent pieces 

 were all somewhat broader. ' 



The Plastron of an embryo Chelonia midas 6 again was proportionately larger, plate iv, fig. 2. Its 



1 The references here are to Rathke's own plates; in my 12th Plate figs. 10 17 are copied from 

 those given by Rathke, and to these references are given in foot-notes. W. K. P. 



2 See Plate XII, fig. 11, th. a. 3 See Plate XII, fig. 12, cl., p. 1. 1. 

 4 See Plate XII, fig. 14. 6 See Plate XII, fig. 10. 



