Gjracantlius tuberculatus and Cladodus mirabilis. 261 



This gentleman, however, seems to confound Diplodus with 

 these dermal tubercles, 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 communicationf 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 asso- 

 ciated 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 mar- 

 gin, which gradually thins out. It measures in the trans- 

 verse 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 ; con- 

 sequently 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 appear- 

 ance, with coarse radiating 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 XVII. 



Fig. 1. View of Pleurodits Rankinii, natural size : a, head ; b, teeth ; 



c c, thoracic expansions ; (/, dorsal spine ; e, counter slab, on 

 : which the left thoracic expansion is preserved, and which is 



represented as if seen through. 



Lanarkshire Coal-field, and on Ctenacanthus hybodoides,''^ Trans, GeoL 

 Soc. Glasgow, vol. iv. pt. 1. pp. 57-59. 



* " 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. 



