20 ANURA CHAP. 



not fused with the rest of the vertebrae. This important dis- 

 covery has been made by Boulenger, but Stannius had previously 

 mentioned a specimen of Pelobates in which the second and 

 fourth vertebrae are biconvex, the third, sixth, and eighth bicon- 

 cave. Moreover, since the sacral vertebra, generally the ninth, 

 in all the Anura is invariably biconvex, the eighth being 

 biconcave in the procoelous families, opisthocoelous like the 

 remaining seven vertebrae in the other families, it is not 

 difficult to imagine that in the Anura the production of pro- or 

 opistho-coelous vertebrae depends simply upon the centra or 

 articulating knobs happening to fuse either with the hind or the 

 front end of the vertebrae. This must of course ultimately be 

 determined by a mechanical problem of motion. 



A second type of the vertebrae amongst the Anura is the 

 epichordal type, an exaggeration in degree of the notocentrous 

 tendencies of the more usual perichordal arrangement. It shows, 

 namely, the almost complete suppression of all the ventral 

 cartilaginous elements, so that the chorda remains for a long 

 time on the ventral surface of the axial column in the shape of 

 a flattened longitudinal band. These two types are not un- 

 connected. The suppression of the ventral elements applies 

 most typically to the trunk region, while hypochordal cartilage 

 exists in the anterior cervical vertebrae, and above all in the 

 coccyx. Typically epichordal are the vertebrae of Pipa, Xeno- 

 pus, Bombinator, Pelobates, Discoglossus and Alytes. It is 

 significant that the epichordal often coincide with opisthocoelous 

 vertebrae, and still more suggestive is the fact that Bombinator 

 is eminently aquatic, Pipa and Xenopus entirely so, having lost 

 the tympanum, at least externally. The epichordal feature is 

 not necessarily indicative of relationship. It has probably been 

 developed independently in various groups, in correlation with a 

 resumption of aquatic life. Various genera of Pelobatidae and 

 most likely some Cystignathidae, e.g. Pseudis, will not improbably 

 connect the two types and their several correlated features, for 

 instance, the frequent reduction of the tympanic cavity. 



The os coccygeum has retained rather primitive features in 

 so far as much dorsal and ventral cartilage is developed ; but this 

 has almost entirely lost its metameric arrangement, and the 

 posterior half of the coccyx is formed chiefly by the ventral mass 

 of cartilage, while the dorsal elements are more or less reduced. 



