556 PEOy. T. W. BRIDGE ON THE MESIAL 



There are indications of the concentration of fin-rays at the 

 anterior end of the fin in the presence of two rays in excess of 

 tlie number of radial elements. 



Chaea.ciis'idje. 



Citharinus Geoff royi. 



Dorsal Jin. — There are sixteen radial elements supporting 

 nineteen dermal fin-rays. The radial elements (PI. XXI. fig. 17) 

 are all bisegmental, consisting of proximal {p.s) and. distal {d.s.) 

 segments, with no trace of mesial segments or of the postero- 

 superior processes which so often take their place. The distal 

 segments are somewhat interesting inasmuch as they illustrate a 

 further stage in the gradual conversion of the segment into the 

 hook-shaped distal segment of many Acanthopterygian Teleosts. 

 The distal segment of the first radial element (r.e.\ d.s.) is larger 

 than any of the others, and in the form of an elongated and 

 somewhat quadrate ossicle articulates with the distal end of its 

 proximal «egment (^.s.), and also partially overlaps the corre- 

 sponding extremity of the next proximal segment and suturally 

 articulates with its distal segment. The remainiug distal seg- 

 ments are somewhat smaller, and many of them exhibit traces of 

 a median longitudinal suture, but all of them have similar 

 relations to their own and succeeding proximal segments as well 

 as to contiguous distal segments. Excluding the first, each 

 distal segment has on its lateral surfaces, near the anterior end 

 of the segment, a concavity so deep that the two nearly meet in 

 the centre of the segment (fig. 17). These concavities or sockets 

 are for the reception of the condylar projections from the inner 

 surfaces of the cleft basal end of a fin-ray (see dorsal Y\ew, 

 fig. 18), and hence the mode of articulation of the two structures 

 assumes a further extension of the " peg-and-socket " joint already 

 indicated in the case of Conger. A slight extension of this 

 modification in the direction of extending the inward growth of 

 the two condylar projections of the fin-ray so that they meet and 

 fuse, while at the same time, the posterior end of each distal seg- 

 ment becomes contracted and curved into a hook, and the charac- 

 teristic "chain-link" articulation of so many Acanthopterygii 

 is easily reached. The distal segment of the first radial element 

 differs from the rest in having three pairs of lateral sockets, the 

 last pair, however, being in part formed by the hinder portion of 



