CHELONIA. 



Thoracic segment of the skeleton of a Bird. 

 Fig. 5. 



Geoffroy St. Hilaire, whose views are generally adopted, was guided in his deter- 

 mination of the parts of the plastron by the 

 analogy of the skeleton of the bird : which 

 analogy may be illustrated by the subjoined dia- 

 grams of corresponding segments of the thorax 

 of a bird (fig. 4) and of a tortoise (fig. 5). In 

 both figures c is the centrum or vertebral body ; 

 ns the neural arch and spine ; compressed in the 

 bird, depressed and laterally expanded, accord- 

 ing to Geoffroy, in the tortoise ; pi the pleura- 

 pophysis, or vertebral rib, expanded in the 

 tortoise, and with its broad tubercle articu- 

 lating with the expanded spine ; k, h' in fig. 5, 

 answers to h in fig. 4, and is the hsemapo- 

 physis (sternal rib, or ossified cartilage of the 

 rib) ; h, hs in fig. 5, is hs in fig. 4, 

 i. e. exclusively a sternum, with the 

 entosternal piece, hs, developed hori- 

 zontally in the tortoise, and vertically 

 in the bird. "Yhe prima facie simplicity 

 of this view has imposed upon most 

 comparative anatomists : and yet 

 there are other vertebrate animals 

 more nearly allied to the Chelonia than 

 birds, and with which, therefore, 

 comparison should have been insti- 

 tuted before general consent was 

 yielded to the Geoffroyan hypothesis. 

 If, e. g. we take the segment of a crocodile's 

 skeleton (fig. 6) corresponding wdth that of the 

 tortoise (fig. 5), the comparison will yield the 

 following interpretation : in both figures c is 

 the centrum : ns the neural arch and spine, 

 with d the diapophysis ; sc a median dermal 

 bony plate (connate with ns in the tortoise) ; 

 pi the pleurapophysis ; sc sc lateral dermal bony 

 plates (connate with pi in the tortoise) ; h, h' 

 in fig. 5, answers to // in fig. 6, an intercalated, 

 semi-ossified piece between pd and h in the 

 crocodile ; h, hs in fig. 5, answers to h, the 



hsemapophysis in the crocodile ; and hs in Thoracic segment of tlie skeleton of a Crocodile. 



fig. 5, exclusively represents hs, the sternum in the crocodile. 



hs JiA- 



Thoracic segment of the skeleton of a Tortoise. 



Fiff. 0. 



