128 VERTEBRA OF THE TORTOISE. 



The want of concordance with the vertebral ribs, or 

 " pleurapophyses," arising from the increased number of 

 the marginal pieces, favors the idea of their being dermal 

 ossifications, such peripheral elements being more sub- 

 ject to vegetative division and multiplication than the 

 haemapophyses : the absence of the marginal pieces in 

 the trionyx gives additional support to the same view. 

 The median piece, S, is here regarded as a h^mal spine : 

 it is called " entosternum." The parial pieces of the plas- 



Fig. 23. 



SEGMENT OF CAHAPACE AND PLASTRON. 



tron are the "haemapophyses" connate with expanded 

 dermal ossifications, and have received the following spe- 

 cial names : es, " episternal ;" hs, " hyosternal ;" ^:>s, " hy- 

 posternal ;" xs, " xiphisternal." 



In some extinct chelonia, the number of these lateral 

 elements of the plastron is increased by an intercalated 

 pair which I have called " mesosternals." In the figure 

 of the segment, as modified to form the carapace and 

 plastron (Cut 23), the nature of the bones is indicated by 

 the letters according to the explanation given of the 

 archetype vertebrae (Fig. 5, p. 29), the dermal superad- 

 ditions being marked sc. 



