106 ON THE MOKPHOLOGICAL CONSTITUTION OF 



its corresponding fin-ray, as consisting of a right and left 

 actinapophysis mesially united — that is, to consider the right 

 and left halves of which they consist in the young fish as 

 fundamental elements of opposite sides of the body. This 

 view of the actinapophyseal character of the bones of the 

 mesial fins appears to be supported by the occurrence of double 

 anal and caudal fins in monstrous fishes, and also by the so- 

 called urohyal bone. The relations of this bone appear to me 

 to indicate that it is not referable to the basohyal elements of 

 the arch, but to the actinapophyseal. I recognise it as con- 

 sistmg of two of these elements fused together at the mesial 

 plane. 



I am further supported in the view which I take of the 

 actinapophyseal character of the inter-spinous bones and mesial 

 fin-rays, by the well-known and hitherto unexplained antero- 

 posterior duplicity which they exhibit in certain fishes. In 

 the Pleuronectidffi, for instance, the inter-spinous bones are 

 attached in pairs, one bone in front and another behind each 

 spinous process. In these instances I conceive we have ex- 

 amples of mesial anterior and posterior actinapophyses in each 

 sclerotome. The corresponding fin-rays are, it is true, alternate, 

 but this does not affect the general principle, when we keep 

 in view the remarkable antero-posterior movements of certain 

 elements of the sclerome discovered by Eemak in the embryo, 

 and the highly-important observations of Professor Owen with 

 reference to the alternations of some of the elements of the 

 spine in certain reptiles and birds — alternations undoubtedly 

 referable to movements of the kind discovered by Eemak. 



In the head actinapophyseal elements are generally bar- 

 like, or more or less flattened from without inwards. From 

 the peculiar forms assumed by these elements in the head, an 

 anterior actinapophysis of one sclerotome may meet a posterior 

 one from the sclerotome in front, so as to form together a bar- 

 like or flattened bridge, or buttress, between the two. These 



