Mr. T. Stock on Ctenoptjcliius pectinatus, Ag. 257 



pointed out*, every gradation exists between the broad forms, 

 which Agassiz called (7. denticulatus^ and the short, named 

 by him C. lyectinatus. I have endeavoured to represent 

 (PI. YIII. figs. 5-1 7) t a series which connects the two ex- 

 tremes. A consideration of the figures will show that there 

 is a considerable range of variation, not only in the width of 

 the specimens, but in the extent to which the roots are deve- 

 loped and the denticles pointed and fasciculated. 



In the forms represented by figs. 6, 12, and 17, the den- 

 ticles are blunt. In the specimen shown in fig. 12 and in 

 one of those seen on the slab, fig. 17, the concave surface is 

 represented ; but in these cases the concavity is slight and the 

 denticles are very little separated at their apices. The blunt 

 appearance is not due to abrasion. The whole of the speci- 

 mens in fig. 17 are bluntly denticulated, and presumably 

 belonged to the same individual, the only instance of the kind 

 that has as yet occurred to me. It seems more reasonable to 

 believe that all of these obtuse forms are indicative of indi- 

 vidual variation rather than of specific dilference. The 

 greatest amount of fasciculation may be observed on the fine 

 example shown at fig. 16, the denticles of which tend to 

 arrange themselves in groups of twos and threes. The speci- 

 men is somewhat fractured ; and I have slighfly restored the 

 fang-like processes. In fig. 13 there is very little, if any, fasci- 

 culation. In fig. 15, an imperfect specimen, the free area is 

 low and very straight. Figs. 10 and 14 represent examples 

 of the ordinary type, but viewed on their concave aspects. 

 Fig. 10 has an abnormally prolonged base. Most of the speci- 

 mens give evidence of the production of the covered area into 

 fang-like extensions. 



Messrs. Hancock and Atthey suggested:]: that these plates 

 might be dermal appendages. The more generally received 

 view appears to be that they are, as Agassiz thought. Sela- 

 chian teeth. The unusual prolongation of the base into roots 

 or fangs is nevertheless paralleled in e. g. Pohjrhizodus ^ a 

 Selachian tooth ; and the liistological characters do not point 

 decisively either way. 



* "Notes on the Hemains of some Reptiles and Fishes from tho Shales 

 of the Northumberland Coal-Held," Nat. 11. Trans. Northumberland and 

 Durham, vol. iii. part i. p. 115 (18G9). 



t The figures are all of the natural size, and are dra'um from specimens 

 obtained at Loauliead in the Carboniferous-Limestone series, 



X Loc. cit. 



