II ^■I-■,RTK^.RAI. cou'Mx 23 



diii]»ophyscs of all those vertebrae ai'i' uiiiled into one broad plute 

 to which the ilia are attaclied. Lastly, in JfytnenocJiirus the 

 first sacral is the sixth vertebra, and this creature has thei'el)y 

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



This shifting forwards of the iliac attachment implies the 

 conversion of original trunk into sacr.d vi^-tebrae, and the 

 original sacral vertebra itself becomes ultimately added to the 

 urostyle. The second sacial. tlie tentli of J'r/obcdes, the ninth 

 of Fipa, and the tenth on tiie right side of the abnormal 

 Boriihinafor, are still in a transitional stage of conversion. In 

 ] )iscoglossidae the tenth is already a lyi)i('al post-sacral vertebra, 

 and is added to the' coccvx, but it still retains distinct, tliouuh 

 short, diapophyses. In the majority of the Aiiura the tenth 

 vertebra has lost these processes, and its once separate nature is 

 visible in young specimens only. In Jhnnhi iiafur even tlie 

 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 coccvx, ami that the ilium was attached nmch 

 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 ril)s 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 recimstructing the 

 vertebral column of the Anura, by dissolving the coccyx into 

 about a doicen 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 ol" 

 the axial continuation Itehind 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 ol' the cartilaginous elements into a continuous 

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

 hinder portion of tlic tail uf Dipnoi and Crossopterygii, and in 

 the tail-end of most I'rodela, 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. 



