66 MONOGRAPH OF DURA DEN. 



the most posterior and largest of the plates on each side. The hinder edge of 

 this is overlapped by the lowest of the opercular bones, which exhibits a well- 

 marked ridged sculpture. 



There are faint traces of small teeth in the edges of the mandibles, and in 

 what remains of the maxilla, but no large tooth is anywhere visible. The 

 posterior edges of the large jugular plates overlap the anterior portion of the 

 bones of the pectoral arch, which exhibit a reticulated sculpture, whose 

 meshes are elongated in the direction of the long axis of the bone. 



Tbe scales, of which about twenty-four series are visible, diverge from the 

 median line in the ordinary way. They are rhomboidal, and have an aver- 

 age short diameter of one-sixth of an inch ; but they are somewhat larger on 

 the anterior part of the ventral surface, than on the posterior part of the same 

 surface, and at the side of the body than on the belly. 



The angles of each scale (J, B) are slightly rounded off. Along the anterior 

 and the inner sides, from a third to two-fifths of the outer surface of the scale 

 is smooth, or marked only by radiating and concentric striae, being over- 

 lapped by the edges of the adjacent scales. The rest of the outer surface is 

 beautifully and variously sculptured. In the anterior part of the ventral 

 region the sculpture, for the most part, takes the form of strong, more or less 

 longitudinal, thick ridges ; but, posteriorly, these become superseded by a 

 pitted or reticulated structure. The weaker parts of the scale breaking away 

 more readily than the others, their free margins often appear toothed or irregular. 



No fins are distinctly definable in this specimen, but there is a broken patch 

 on the right side, where the right pectoral should have been, and a few fin 

 rays are traceable in the place of the left pectoral. 



From the totally different views of the body presented by these two speci- 

 mens, the only points of comparison between them are furnished by the 

 scales, which exhibit a general correspondence in form and size, while the 

 middle ventral scales of the second specimen would leave impressions, not un- 

 like those visible on the matrix in the middle lateral region of the first speci- 

 men. It is certainly a little difficult to reconcile the thick and depressed form 

 of the second specimen, with the apparently much thinner and more compressed 

 figure of the first ; but it must be remembered that of this specimen we have 

 only half, and that the direction in which pressure has been exerted may have 

 greatly modified the forms of both specimens. 



Again, the impressions and the general aspect of the scales in the first 

 specimen do not tally exactly with what might be expected from the per- 

 fectly presei'ved corresponding organs in the second ; but then it must be 

 recollected that the precisely corresponding scales in the two fishes cannot be 

 compared. 



On the whole, although not by any means satisfied as to the specific 

 identity of the two fish, I think it better to assume it until further evidence 

 proves the contrary.* 



* Since the above was written, I have seen specimens of Glyptolcemus in the collection of 

 Lord Kinnaird, at Rossie Priory, which leave no doxibt on ruy mind as to the correctness of 

 this assumption. In one of these specimens the edge of the maxilla exhibits a single series 

 of small and slender teeth. 



