ON GYRACANTHUS TUBERCULATUS. 421 



Gyracanthus tuberculattjs, Agassiz; and Cladodus mira- 

 bilis, Agassiz. 



We believe we were the first to point out that certain minute 

 bodies found associated with the remains of these two species 

 are dermal tubercles.* When we wrote our remarks on the 

 subject we described two forms of these peculiar bodies — one 

 considerably larger than the other, and having from four to 

 seven cusps with carinse on their convex surfaces, the smaller 

 form having only two or three smooth points. And we thought 

 both varieties belonged to Gyracanthus, having found the large 

 scattered amidst the small form (which latter was by far the 

 more numerous), and both associated with the spines of that fish 

 and with the teeth of Cladodus. We have long been satisfied, 

 however, that this was a mistake, and that, while the small form 

 is the dermal tubercle of Gyracanthus, the large variety is that 

 of Cladodus. This is satisfactorily proved by numerous speci- 

 mens in our possession, in which the small variety unmixed with 

 the other is associated in large patches with the spines and other 

 remains of Gyracanthus; while the large form has occurred on 

 several occasions, unaccompanied by the small variety, on the 

 same slab with the teeth of Cladodus and the spines of Ctena- 

 canthus hybodoides. This has so frequently happened now, that 

 it is impossible any longer to question the fact that the two forms 

 belong respectively to these two large Selachians. And we are 

 also satisfied that the so-called tooth Mitrodus quadricornis of 

 Owen is the larger form of these dermal tubercles, as we origi- 

 nally asserted, and consequently belongs to Cladodus or Ctena- 

 canthus, and not to Gyracanthus, as we at first thought. 



We have much pleasure in observing that the dermal nature 

 of these minute spinous bodies has recently been confirmed by 

 the researches of Mr. James Thomson, of Glasgow, who has 

 found the large form associated with the teeth of Cladodus mirabi- 

 lis and the spines of Ctenacanthus hybodoides. t This gentleman 



* See paper entitled " Notes on the Remains of some Reptiles and Fishes from the Shales 

 of the Northumberland Coal-Field," Ann. Nat. Hist., Ser. 4, Vol. I., p. 370. 



t See paper entitled " On a Specimen of Acanthodes Wardii from the Lanarkshire Coal- 

 Field, and on Ctenacanthus hybodoides," Trans. Geol. Soc. Glasgow, Vol. IV., pp. 57-59. 



