194 RANA. 



surface near the base, to a continuous alveolar groove defended by an 

 external wall of bone. In the common frog {Rana temporaria), there 

 are about forty of these teeth on each side of the upper jaw, of which 

 eight are supported by the intermaxillary bones. In the Rana esculenta, 

 (PI. 62, fig. 8), there are the same number of intermaxillary, but a 

 greater number of maxillary teeth. In the great bull-frog, {Rana 

 pipiens) there are ten or twelve intermaxillary teeth, followed by 

 between sixty and seventy maxillary teeth (PL 62, fig. 10). The 

 largest teeth are placed in the anterior third of the series, whence 

 they progressively diminish in size towards the two extremities. The 

 vomerine bones each support a short transverse row of four or five 

 small teeth. The base of the teeth, besides being confluent with 

 the bone, are, from their close contiguity, frequently anchylosed to 

 one another : they are sometimes thus conjoined to near the apex. 

 Where the larger teeth are situated, the alternate ones are commonly 

 found to be displaced. The germs of the successional teeth are 

 developed in a groove of the mucous membrane of the mouth which 

 covers the inner side of the basis of the teeth ; as they increase in size 

 they press upon and cause absorption of the contiguous surface of the 

 base of the old teeth, and thus finally displace them and become in 

 their turn anchylosed to the bone, then undermined and shed. As a 

 general rule the toads {Bufonidce) are edentulous, but among the 

 Bombinatores, or those species which are termed ' earless,' from 

 having the tympanum concealed under the skin, the subgenus Hyla- 

 dactylus has teeth upon the vomer and Sclerophrys has teeth on 

 both the intermaxillary and maxillary bones. (1) 



In microscopic structure, the teeth of the existing Batrachians 

 like those of most Saurians, correspond with the simple mamma- 

 lian teeth. 



If a longitudinal section of the tooth of a frog, including a portion 

 of the alveolar plate to which it is anchylosed, be examined by trans- 

 mitted light under a half-inch objective, a narrow transverse line will 

 be seen inflected inwards from both the external and internal side of 

 its base, defining the Umits of the tooth and bone. Below this line 

 the bone is characterized by numerous large, oblong, radiated cells, 



(1) M. Tschudi, 1. c. p. 3. 



I 



