EEMAINS OF REPTILES AND FISHES. Ill 



tubercles. In size, proportion, and form it agrees exactly with 

 them ; and in the minute structure there is no difference what- 

 ever, as is demonstrated by the numerous sections of them which 

 we have had the advantage of examining. This " minnow," 

 then, of our shales is found to be identical with Gyracanthus 

 tuherculatim, perhaps the largest fish of the Coal-Measures. 



In the figure of Mitrodus only a small portion of the denticles 

 is shown ; the points, being strongly recurved, are necessarily 

 cut away in such a section as that represented. It is only the 

 base of the toothlets that Professor Owen has seen ; and, conse- 

 quently, his knowledge of the true form must be very imperfect. 

 The angles represented at the margin of the denticles indicate 

 the external ridges described above. 



DiPLODUs GiBBOSus, Agossiz. 



This is a common fossil at Newsham and Cramlington, and is 

 usually found in connexion with a thick granular layer of a sub- 

 stance resembling shagreen, large patches of which frequently 

 occur studded all over with it. One such patch has been ob- 

 tained, which measured fifteen inches long, and about seven 

 inches wide. On this the Diplodi are comparatively few in num- 

 ber, and are scattered about. But in another patch, of which 

 there are fifty- six square inches, they are very numerous, and 

 are crowded together without order. 



There can be little doubt that these shagreen-like patches are 

 the remains of the skin of some large fish, and that the Dijjlodi 

 are dermal tubercles in connexion with it, and analogous to the 

 spinous tubercles of the Eays. At the same time it must be ad- 

 mitted that it is possible enough that the larger specimens may 

 have clothed the lips or jaws with a spinous pavement resem- 

 bling in arrangement the oral armature of the Eays or Cestra- 

 cionts ; or they may have ranged along the back or sides of the 

 body in serial order, as the dermal spines frequently do in the 

 Kays ; or perhaps they may have been scattered here and there 

 among the smaller ones, as is not unfrequently the case with 

 such tubercles. 



Diplodus has usually three recurved spines, two being large, 



