598 PEOr. T. W. BEIDGE ON" THE MESIAL 



base of the spinose ray. As previously mentioned in the case of 

 Conger, Oitharinus, and Solocentrum, the book-lite process pro- 

 bably owes its formation to a further modification of the " peg- 

 and-socket " method of articulation. Examples of this form of 

 " chain-link " articulation are to be found in the Scombridse, 

 Percidse, Berycidae, aud Sparidse. 



(h) The suppression of the series of mesial and distal seg- 

 ments and the extension of the bony tubercle already mentioned 

 above in the form of a loop forwards and downwards to its 

 fusion with the distal end of the same proximal segment at a 

 point more anterior to its origin — the loop, as before, traversing 

 a foramen in the base of the spine. In this case there can be 

 little doubt that the loop owes its formation to the growth of the 

 bouy tubercle by the ossification of the interossicular ligament. 

 The Mugilidse and the Labridse furnish examples of this variety 

 of " chain-link." 



{c) The ingrowth of tubercles from the inner surfaces of the 

 basal halves of a cleft spine through the distal margin of a 

 proximal segment and their subsequent mesial union. This 

 method is probably due to a modification of the " peg-and- 

 socket " joint, except that the ingrowing tubercles perforate the 

 superior margin of a proximal radial segment instead of a 

 distal segment. Examples of this method of articulation may 

 be found in the anterior and usually supernumerary spinose rays 

 of the dorsal or anal fins of the Percidse {Mesoprion), Sparidse 

 {Pagellus), Scombridse {Scomber), Carangidse {Garanx), and 

 Mugilidse {Mugil). It is possible, however, in some cases, as 

 in the particular instance of the second and third anal spines of 

 Tlolocentrum, that the ossification of the interossicular ligament, 

 by which the diatal radial segments are connected with their own 

 and with immediately adjacent proximal segments, may contribute 

 to the formation of the bony loops. 



The " hook-link " is, so to speak, an incipient stage in develop- 

 ment of a form of chain-link (S), and is associated with the 

 suppression of both the mesial and radial segments and the 

 growth of the bony tubercle above mentioned in the form of a 

 hook through a foramen in the base of a fin-ray, but without 

 again uniting with the proximal segment to which it belongs. 

 In this form of joint, as previously pointed out, each ray or spine 

 is solely supported by the proximal radial segment immediately 

 posterior to that to which it rightly pertains, as, for example, 

 in the dorsal fin of the Blenniidse. 



