422 EXPLANATION OF THE PLATES. 



however, seems to confound Diplodus with these dermal tu- 

 bercles, and to consider the remains of the semicartilaginous 

 skeleton to be shagreen. It is to Professor Williamson that we 

 owe the discovery of the true nature of this peculiar substance, 

 who clearly proves it to be the remains of what he terms the 

 chondriform bone or semicartilaginous skeleton.* 



In a former communication! we described a large triangular 

 bone associated with the spines of Gyracanthus as one of the 

 carpals. We have now to notice a second carpal, several of 

 which have occurred on the same slabs with the spines and 

 triangular bones. In one instance the two spines are associated 

 with one triangular bone and two of our second carpal. This 

 second form is probably the inner carpal: it is a broad, flat 

 bone, irregularly bilobed, or somewhat reniform, with one of 

 the lobes produced and the external margin straightened ; the 

 convex border is a little flattened, angulated, and thickened ; 

 thence the bony fibres radiate to the opposite or lobed margin, 

 which gradually thins out. It measures in the transverse or 

 longest diameter eight inches and a quarter, and in length, from 

 the thickened to the thin margin, two inches and a half. The 

 former we take to be the proximal margin; consequently the 

 thin opposite edge will give support to the fin. The texture of 

 this bone is quite similar to that of the large triangular carpal ; 

 namely, it is of a semicartilaginous appearance, with coarse ra- 

 diating fibres extending from margin to margin. 



Helodus simplex, Agassiz. 

 We take this opportunity to announce the occurrence of this 

 strange form of tooth at Prestwick, Northumberland. Only a 

 single specimen has been found ; and we believe this to be the 

 first that has been obtained in the district. 



EXPLANATION OF THE PLATES. 

 PLATE XV. 

 Fig. 1. View of Pleurodus Rankinii, natural size; a, head; 6, teeth; c c, 

 thoracic expansions ; d, dorsal spine ; e, counter slab, on which 



* "Investigations into the Structure and Development of the Scales and Bones of 

 Fishes," by W. C. Williamson, Philosophical Transactions, 1851, Pt. 1, pp. 669-679. 

 t Ann. Nat. Hist., Ser. 4, Vol. I., p. 369. 



