VI.] THE COMMON FROG. 



71 



In a certain North African Salamander named 

 Pleiiroddcs the ribs are not only elongated, but their 

 apices, if they do not actually perforate the skin, are 

 so prominent as to seem to do so, when the finger is 

 drawn from behind forwards along the side of the 

 animal's body. 



The several joints of the backbone are connected 

 together by surfaces which are not the same on both 

 the anterior and posterior sides of the centrum, or 

 body, of the same vertebra. Each of the first seven 

 vertebrae is furnished with a round prominence, or 

 liead, on the hinder side of its centrum, and each of 

 the precoccygeal vertebrae, except the first and last, 

 has the anterior surface of its centrum excavated as 

 a cup for the reception of the ball of the hinder 

 surface of the vertebra next in front. The first 

 vertebra has in front two concavities, side by side, 

 to articulate with the skull. The eighth vertebra has 

 a concavity at each end of its " body." The ninth 

 vertebra has a body provided with a single convexity 

 in front and a double convexity behind, to articulate 

 with the concavities placed side by side on the front 

 end of the coccyx. 



These arrangements are not constant in the frog's 

 order, still less in its class. In Bovibinatei^ and Pipa 

 the vertebrae are concave behind each centrum, instead 

 of in front : and the same is the case in Salamandra. 

 In many tailed Batrachians the vertebrae are bicon- 

 cave, as e.g. in Spelerpes, AvipJiiuma, Proteus, and 

 Siren. 



The biconcave shape is an approximation towards 

 the condition which is almost universal in bony fishes, 



