from the Shales of the Northumberland Coalfield. 369 



ference, is only seven inches long, including the portion buried 

 beneath the skin. 



All this seems to demonstrate, beyond doubt, that these are 

 really paired spines, most probably pectoral; and from this 

 wearing we may fairly assume that Gyracanthus was a ground- 

 fish, and that the spines assisted its motions at the bottom of 

 the water. 



The straight spines, or those which are not laterally bent, 

 are all regularly arched from before backwards ; and their 

 distal or pointed extremities are all perfect, not being in the 

 least degree worn. These are apparently dorsal spines ; and 

 that there is only one of such in each fish seems probable from 

 the fact that they occur in the ratio of one to two of the paired 

 spines, as shown by our previous division of the seventy-one 

 specimens. 



The dorsal spines are considerably smaller than the paired 

 ones ; they are more compressed, and the posterior denticu- 

 lated keel is more strongly developed ; the extreme point is 

 smooth, compressed, and rounded in front. The largest are 

 about eleven inches long, and three and one-quarter inches in 

 circumference at the thickest part. The paired spines are 

 fifteen or sixteen inches in length, and upwards of six and a 

 half inches in circumference. 



One or two specimens of the species denominated O. for- 

 mosus have likewise occurred ; and as the same spine of G. 

 tuherculatus is occasionally found with both tuberculated and 

 smooth ridges, the former can scarcely be considered a good 

 species. M. Agassiz's figure of G.formosus'^^ like G. tuher- 

 culatus^ is laterally bent. 



Large flat triangular bones are frequently found associated 

 with the spines, measuring sometimes eight and a half inches 

 long and six and a half inches broad at the widest part. 

 Their structure is very open ; and as they are seldom well pre- 

 served, they are probably only imperfectly ossified ; the bone- 

 fibre radiates from the apex to the expanded base. There can 

 be little doubt that these are carpal bones, similar to those in 

 connexion with the pectoral fins in the Sharks and Dogfishes. 

 This bone is thickest at the apex, which is rounded, and thins 

 out towards the distal expanded margin or base. The large 

 longitudinal groove at the root of the spine probably corre- 

 sponds to the lower or anterior margin of this bone ; or it may 

 be that it was fitted to a lower carpal which was coadjusted 

 to this bone but, being entirely cartilaginous, has disappeared. 

 However this may be, it can scarcely be doubted that this 



* Poissons Fossiles, vol. iii. tab. 6. figs. 4, 5, 6. 



