CIIELONIA. 



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 ; h, li. in fig. 5, 

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

 physis (sternal rib, or ossified cartilage of the 

 rib) ; h, lis in fig. 5, is ha in fig. 4, 

 i. e. exclusively a sternum, with the 

 entosternal piece, hs , developed hori- 

 zontally in the tortoise, and vertically 

 in the bird. 'Yh.Q 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- 



Fis. i. 



Thoracic sej^meut of tlie skeleton of a Bird. 



Fig. 5. 



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 with 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, U 



in fig. 5, answers to li! in fig. 6, an intercalated, 



semi-ossified piece between pd and k in the 



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



haemapophysis in the crocodile ; and hs in 



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



h s lit 



Thoracic segment of the skeleton of a Tortoise. 



Fiir. (■). 



Thoracic segment of the skeleton of a Crocodili 



