FINS OF aANOIDS AND TELEOSTS. 



597 



of a fin, not only in the genera above mentioned, but in the 

 Pleuronectidae, Gymnotidse, Lophobranchii, and Plectognatbi, 

 and very generally also in the feebler rays which constitute the 

 hinder part of the fia in such Teleosts as possess a distal series 

 of radial segments. With an increase in the size of the soft 

 fin-rays towards the central and anterior portions of a fin, the 

 proximal extremities of the cleft base of a ray may become 

 enlarged and terminate in two lateral basal condyles which 

 acquire a definite articulation with facets on the anterior portion 

 of the distal end of the next succeeding proximal segment, in 

 addition to its normal relations with its own distal radial seg- 

 ment ; while it may not infrequently be the case that a firmer 

 connexion between the distal segment and its fin-ray is brought 

 about, by the development of two in-growing tubercular or peg- 

 like processes from the inner surfaces of the cleft base of a ray, 

 which fit into corresponding sockets on the lateral surfaces of the 

 distal segment (peg-and-socket joint), as, for example, in Citha- 

 ri7ius and Conge7\ In the case of the spinose and often massive 

 rays of the anterior portion of a fin, the methods of articulation 

 are many and various. Excluding the Acanthopterygii and 

 dealing first with the Physostomi, the base of a spine, by the 

 secondary closure of the basal cleft, may become converted into 

 a transversely extended condyle articulating, in the absence of 

 a distal segment, with a suitably modified surface or groove on 

 the distal extremity of the proximal radial segment, and, in 

 addition, possessing also a " hook-link " or even a "chain-link " 

 connexion with the same segment, as is the case, for example, wirh 

 the defensive spines of many Siluridse ; or the spines, retaining 

 their cleft bases, may simply clip the dorsal margin of the segment 

 {e. g. the guard-spines of the Siluridse) ; or, finally, their method 

 of articulation may be precisely similar to that of the larger soft 

 rays, as in the serrated defensive spines of Cyprinus and Barhus. 

 The most characteristic methods of connexion between the 

 spinose rays and their radial elements are, however, the " chain- 

 link " and " hook-link " articulations of the anterior dorsal tin of 

 the Acanthopterygii. 



" Chain-link " articulations may be formed in several ways : — 

 (a) By the formation of a hook-like bony process from the 

 hinder margin of a distal radial segment, which extends back- 

 wards to a sutural or a firm ligamentous connexion with a buny 

 tubercle on the distal end of the next succeeding proximal 

 segment, the bony loop thus formed traversing a foramen in the 



