FINS OF GANOIDS AND TELEOSTS. 561 



between successive sections of the osseous groove serve for the 

 transmission of the elevator and depressor muscles of the spines. 

 Each distal segment (PL XXII. figs. 21, 22, d.s.) suturally articu- 

 lates with the hinder margin of the distal end of the corresponding 

 proximal segment, and consists of (a) a central nodular portion, 

 (5) the lateral wings already mentioned, and (c) a hook-shaped pro- 

 longation (A) from the centre of its hinder margin, which, after 

 curving backwards and a little downwards, becomes tirmly con- 

 nected by ligament with an osseous tubercle {t) on the adjacent 

 distal end of the proximal segment of the next succeeding radial 

 element in such a way that the hook and tubercle together form 

 a bony link or loop. Posteriorly, the distal segment articu- 

 lates with the anterior of the two facets with which the distal 

 end of the next succeeding proximal segment is furnished. 



The spinose fin-rays have much the same structure throughout 

 the series. In each case the base of the spine forms a trans- 

 versely elongated condyle for articulation with a similar facet on 

 the distal end of the proximal radial segment, immediately behind 

 that to which it rightly belongs; while above the condyle the 

 base of the spine is perforated by a foramen, through which 

 passes the bony hook formed by the distal segment of its proper 

 radial element, that is, the next anterior element. 



The evolution of this method of articulation between a distal 

 radial segment and a fin-ray is, I believe, an extreme modi- 

 fication of the "peg-and-socket " articulation, the first appearance 

 of which was noted in the Conger and a later stage in Citharinus. 

 The ingrowtlis from the inner surfaces of the originally cleft 

 base of a fin-ray have now^ met and fused, forming a transverse 

 basal condyle for articulation with a proximal radial segment, 

 but above the condyle there is left a foramen through which passes 

 the now contracted and hook-like posterior portion of the distal 

 segment. This mode of articulation is extremely characteristic 

 of many Acanthopterygian Teleosts, and in future will be referred 

 to as a "chain-link," although I may so far anticipate the sequel 

 as to say that " chain-link " articulations may be formed by 

 various methods in diflerent Teleosts. 



The first radial element (PI. XXII. fig. 21, r.e?) supports two 

 spines, of which the first has a chain-link articulation with the distal 

 extremity of the proximal segment, while the second spine, the 

 proper ray to this element, has the normal " chain-link " connexion 

 with the distal segment. It is somewhat difficult to account for 



LINN. JOUEN. — ZOOLOGY, VOL. XXT. 45 



