REPTILES. 135 



azygous piece was relatively much longer and broader, and presented the form of an elongated and not 

 perfectly isosceles triangle, with its point directed backwards. The symmetrical pieces were somewhat 

 broader and thicker than in the young Chelonia virgata. In these embryos, also, the pieces forming the 

 second and third pieces still lay far asunder. The future alse or the Plastron appeared iu the form of 

 three or four dentations or striae running outwards. Similar in their form and position were the 

 several bony pieces constituting the Plastron in young embryos of Chelonians that appeared to me to 

 belong to the species Chelonia midas, only that they were somewhat more slender than in that embryo. 

 As appears from a comparison of the Plastron of these various specimens of Chelonians, the further stages 

 of development must have consisted in the opposite pieces of the second and third pairs sending each a 

 broad process inwards, by the union of which with each other, and with the remaining symmetrical 

 pieces, a complete ring is constructed. Such a process appeared to me to be feebly developed in the 

 second pair of pieces in the young of Chelonia virgata. The development of the Plastron of the embryo 

 Testudo 1 had taken place to about the same extent as in the embryo Chelonia (plate iii, fig. 14). The 

 asymmetrical piece in the former was of similar form, and of about equal size to that of the latter. The 

 symmetrical pieces were likewise upon the whole only moderately broad ; those of the first and fourth 

 pairs were somewhat narrower and altogether smaller than the remainder. From the bony pieces of 

 the second and third pairs a wing extended outwards, of moderate length, perfectly simple, and tapering 

 to a blunt point; not, however, as in the marine Tortoises, horizontally placed, but already, as in adult 

 specimens, turning upwards with a moderately strong arch, and with its extremity directed inwards (from 

 the annular fold of the cutaneous investment separating the belly from the back) until it reached the 

 dorsal wall. Moreover, the bony pieces of the second pair were considerably more distant from those 

 of the third than in the embryo or the young of Chelonia. Moreover the space around the nine bony 

 plates of the Plastron was filled up with connective tissue, and in the centre was the umbilical opening, 

 which was proportionately much larger than in the young of Chelonia. The Plastron in the remaining 

 Tortoises examined was still further developed. Its symmetrical pieces were in contact as they 

 followed one another from before backwards. Nevertheless, in Trionyx ocellatus, but especially in 

 T. gangeticus (pi. vi, fig. 13), 3 and T. eeayptiacus, the pieces constituting the second, third, and fourth 

 pairs were much narrower than in adult specimens of this genus, so that here the whole Plastron only 

 presented a narrow ring, which sent out four horizontally running and only very narrow, but apparently 

 long alffi. In Emys europcea (plate vi, fig. 15), 3 in Emys lutaria, in Terrapene tricarinala, and pro- 

 bably also in Platemys (plate vi, fig. 23), 4 the symmetrical pieces were proportionally smaller than 

 in adult specimens ; so that the Plastron composed of these pieces is only represented in Platemys by 

 a broad ring, and in Emys and Terrapene it allows a series of well-marked spaces, which are connected 

 with one another near the middle line, to be seen. Moreover, in Pentonyx capensis there are present 

 in the Plastron three consecutive spaces of considerable magnitude and of various size, but they are no 

 longer continuous with one another (plate vii, fig. 2). 



From the preceding observations on the development of the Plastron in the Chelonians, the following 

 conclusions may be drawn : 



1. That in all probability the basis of the symmetrical pieces is laid down at an earlier period 

 than that of the azygous pieces. 



2. That the basis (mother-substance) of the symmetrical pieces consists of four cartilaginous [gristly] 

 segments on the two sides of the body, in each of which, at a later period, one bony point is developed. 



3. That the bony pieces of the second and third pairs, like the cartilaginous segments, from 

 which they take their origin at first, lie at a considerable distance from one another; and 



1 See Plate XII, fig. 16. 3 See Plate XII, fig. 13. 



3 See Plate XII, fig. 15. * See Plate XII, fig. 17 



