ON BALISTES VETULA. 55 
that the outer extremities of the rays form a single series, 
those of one tin alternating with those of the other, while 
their bases are kei)t ai)art by the posterior prolongation of 
the eoxa. Thf; rays are all simple ; the spine probably 
re})reseiits two rays, one from eaeh iin. In the eml)ryo 
no doubt at some stages the two ventrals will I)e found en- 
tirely distinet, and in traeing the affinities of the S[)ecies 
backward we may expect them to lead to forms on which 
throughout their lives these fins remain separate. 
