FINS OF GANOIDS AND TELEOSTS. 55H 



the lateral wings at tlie distal end of the third proximal segment, 

 immediately anterior to the origin of its postero-superior process, 

 while the medinn condyle fits into a mesial pit. Above the three 

 condyles, the base of the spine is perforated by an oval foramen 

 through which is prolonged a curious hook-shaped process (h) 

 developed from the anterior or dorsal surface of the postero- 

 superior process, and from the extremity of the hook a stout 

 ligament extends to an insertion into the distal end of the second 

 proximal segment. This hook probably owes its formation to the 

 partial ossification of the strong interossicular ligament which, 

 in the absence of distal segments, passes between the distal ex- 

 tremities of the proximal segments of the first two radial elements, 

 and in other Siluroids, where tlie ligament is completely ossified, 

 gives rise to the characteristic " chain-link " articulation of the 

 defensive spine with its supporting radial element. As the third 

 radial element is already provided with a fin-ray, viz. the first 

 soft ray, the defensive spine must be regarded as the ray normally 

 pertaining to the second element. The " guard-spine " is a simple, 

 short, V-shaped ossicle and, although supported by the second 

 radial element, is really the fin-ray of the first. 



This view of the relations of the anterior fin-rays to their 

 supporting radial elements differs from that given by McMurrich 

 in the case of Amiurus in one or two particulars. According to 

 this writer the radial element of the defensive spine is the third, 

 that of the "guard-spine" being the second, while the fin-ray of the 

 first element is represented by the V-shaped lamina. The reason 

 assigned for the last suggestion is — that what corresponds to the 

 V-sLaped lamina in Amiurus is an ossification in membrane, and 

 ought therefore to be regarded as belonging to the category of 

 fin-rays, inasniuch as the radial elements are always preformed 

 in cartilage. In my opinion this reason is scarcely a conclusive 

 one. The lateral wings of the third proximal segment in Pla- 

 ty stoma are almost certainly formed of membrane-bone, and 

 the same is in all probability true of the produced lateral 

 margins of the first ; but these facts alone are quite insufficient to 

 justify one in regarding such outgrowths as degenerate fin-rays. 

 Moreover, it is admitted by McMurrich that portions of the first 

 and third " interspinalia " in Amiurus are formed of membrane- 

 bone, and yet it is not suggested that such portions represent fin- 

 rays. It seems more reasonable to infer that the partial ossifi- 

 cation of certain proximal segments from membrane is the result 



