570 PBOr. T. "W. BRIDGE ON THE MESIAL 



and proximal segments of two contiguous radial elements, tbe 

 groove being bounded anteriorly by the posterior margin of a 

 distal segment (d.s.), and behind and below by the anterior 

 portion of the distal extremity of the next proximal segment (p.s.). 

 As the second spine is the normal fin-ray of the first radial 

 element, it is evident that the last element has no proper spine 

 of its own, although it contributes to the formation of the groove 

 for the fifth spine. 



Three large T-shaped vestigial radial elements are situated 

 immediately anterior to the first spine-bearing one. 



Posterior Dorsal Jin. — There are ten radial elements and eleven 

 fin-rays. Of the radial elements the first five are bisegmental ; 

 the remainder, owing to the presence of mesial segments, are 

 trisegmental. Behind the last there is a vestigial proximal 

 segment partially fused with the corresponding segment of the 

 antecedent radial element. 



The first fin-ray is a spine, and its mode of articulation with 

 the distal segment of the first radial element affords a further 

 illustration of the method by which an ordinary cubical distal 

 segment may become converted into a " hooked segment," with 

 its characteristic articulation with the perforate base of a fin-ray. 

 The distal segment in question is cubical anteriorly but behind 

 contracts into a short, slightly curved hook-like process. The 

 cleft base of the spine has two ingrowing processes, which, how- 

 ever, do not meet so as to bound a complete basal foramen ; 

 nevertheless, the hooked end of the distal segment fits into this 

 incomplete foramen. The formation of the hook-like process 

 seems, without doubt, to be due to the ingrowth of the two 

 processes of the spine, and the consequent constriction of the 

 posterior half of the segment to the condition of a relatively 

 slender hook, which, however, still retains its normal position in 

 the cleft of the spine. The distal segment of the second radial 

 element is somewhat similar to that of the first, but more closely 

 resembles the ordinary cubical distal segments of the rest of the 

 fin, which are simply clipped by the cleft bases of the fin-rays. 

 The last distal segment supports two feeble fin-rays. 



Anal fin. — Nine radial elements and eleven fin-rays are present 

 in this fin. The fin-supports are similar to those of the posterior 

 dorsal fin, and, as in the latter, certain of them are trisegmental, 

 viz. the sixth to the ninth inclusive, while the remainder are 

 bisegmental. Behind the last of the series there is a vestigial 



