1 82 A NEW CHIM^ROID FIN-SPINE. 



The fossil in question was obtained from the Whitbed of the 

 Portland Stone in a quarry near the parish church, Portland, by 

 Mr. F. J. Barnes, to whom I am indebted for the opportunity of 

 studying the specimen. It is merely the basal part of a bilater- 

 ally symmetrical fin-spine below the point where the posterior 

 denticles begin, and is shown of the natural size in Plate A. 

 As indicated by the transverse sections, its side-to-side measure- 

 ment is greatest at the lower end (Fig. la), where it is widely 

 open behind to receive the edge of the fin. Higher up, where 

 its internal cavity is closed behind (Fig. ib\ the spine is 

 somewhat laterally compressed. Its anterior border is con- 

 stricted into a prominent keel, while its posterior face is sharply 

 excavated by a longitudinal channel. The anterior and lateral 

 faces show that the spine cannot have been deeply inserted in 

 the soft parts of the fish, a narrow lower margin alone having 

 been covered. Its outer surface is ornamented with very coarse 

 rounded tubercles, which are irregular in shape and size, seem 

 to be covered with an extremely thin layer of enamel, and are 

 conspicuous on the anterior keel (Fig. ir). The arrangement 

 of these tubercles is rather indefinite, but there is a tendency for 

 them to follow the longitudinal ridges which result from the 

 texture of the spine. The whole tissue is of a very porous 

 texture. 



The spine thus described must obviously have armed the 

 dorsal fin of a cartilaginous fish ; and the fact that it exhibits no 

 long base of insertion proves that it belongs to a Chimaeroid 

 rather than to an Elasmobranch. The prominent anterior keel, 

 the deep posterior channel, the open texture, and the tendency 

 towards longitudinal ridges on the lateral face suggest com- 

 parisons with the dorsal fin-spine of Ischyodus (Fig. 2.) The 

 remarkable bubbly tuberculation, however, is unique. I am 

 inclined to regard the latter peculiar character as the result of 

 overgrowth in an unusually gigantic form ; and, since the teeth 

 of Ischyodus townscndi from the same geological horizon probably 

 represent an equally large fish, I think it is almost certain that 

 the new spine will eventually prove to belong to this species. 



