II VERTEBRAL COLUMN 23 



diapophyses of all these vertebrae are united into one broad plate 

 to which the ilia are attached. Lastly, in Hymenocliirus the 

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

 reduced the pre-sacral vertebrae to the smallest number known. 



This shifting forwards of the iliac 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 Pelolates, the ninth 

 of Pipa, and the tenth on the right side of the abnormal 

 Bombinator, 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 Bombinator 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 in- 

 dividuality. 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 for- 

 wards, 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 

 exists between the Anura and Urodela. That whole portion of 

 the axial continuation behind the coccyx, more or less coinciding 

 with the position of the vent, is the transitional tail. 



The disappearance of both notochord and spinal cord, and 

 the conversion of the cartilaginous elements into a continuous 

 rod in the case of the os coccygeum, find an analogy in the 

 hinder portion of the tail of Dipnoi and Crossopterygii, and in 

 the tail-end of most Urodela, portions which are not homologous 

 with the os coccygeum. The term urostyle should be restricted 

 to such and similar modifications of the tail-end, and this latter 

 happens to be lost by the Anura during metamorphosis. 



