VARIATION IN TOAD AND ISOPOD. 



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have been described in Bornbinator and Alytes, where the right ilium 

 articulated with the tenth, the left ilium with the ninth vertebra. This 

 shifting forwards of the ilium to the extent of one metamere has been con- 

 tinued further in Pipa, in which the sacrum is formed by the ninth and 

 eighth vertebra, their diapophyses fusing on either side into extra broad 

 wing-like expansions. In old specimens of Palaeobatrachus fritschi the 

 seventh vertebra is in a transitional condition, the ilium being carried by 

 the ninth and eighth, and slightly also by the diapophyses of the seventh 

 vertebra ; and in P. diluvianus the diapophyses of all these vertebrae are 

 united into one broad plate to which the ilia are attached. Lastly, in 

 Hymenochirus the first sacral is the sixth vertebra, and this creature has 

 thereby reduced the presacral vertebrae to the smallest number known. 



" This shifting forwards of the ilia attachment implies the conversion 

 of original trunk into sacral vertebrae, and the original sacral vertebra itself 

 becomes ultimately added to the urostyle. The second sacral, the tenth 

 of Pelobates, the ninth of Pipa, and the tenth on the right side of the 

 abnormal Bornbinator, are still in a transitional stage of conversion. In 

 Discoglossidae the tenth is already a typical post- sacral vertebra, and is 

 added to the coccyx, but it still retains distinct, though short, diapophyses. 

 In the majority of the Anura the tenth vertebra has lost these processes, 

 and its once separate nature is visible in young specimens only. In 

 Bornbinator even the eleventh vertebra is free during the larval stage. In 

 fact the whole coccyx is the result of the fusion of about twelve or more 

 vertebrae, which from behind forwards have lost their individuality. We 

 conclude that originally, in the early Anura, there was no coccyx, and that 

 the ilium was attached much farther back ; and this condition, and the 

 gradual shifting forwards, supply an intelligible cause of the formation of 

 an os coccygeum. The fact that the sacral vertebrae of the Anura possess 

 no traces of ribs, as carriers of the ilia, is also very suggestive. The ilia 

 have shifted into a region, the vertebrae of which had already lost their 

 ribs. By reconstructing the vertebral column of the Anura, by dissolving 

 the coccyx into about a dozen vertebrae, so that originally, say the twenty- 

 first vertebra carried the ilia, we bridge over the enormous gap which 



