ON BALISTES VETULA. 55 



that the outer extremities of the rays form a single series, 

 those of one fin alternating with those of the other, while 

 their bases are kept apart by the posterior prolongation of 

 the coxa. The rays are all simple ; the spine probably 

 represents two rays, one from each fin. In the embryo 

 no doubt at some stages the two ventrals will be found en- 

 tirely distinct, and in tracing the affinities of the species 

 backward we may expect them to lead to forms on which 

 throughout their lives these fins remain separate. 



