416 Proceedings of the Royal Physical Society. 



mistake has certainly been made. So far as I have seen 

 them, the teeth which are found associated with the Coah 

 measure, Ctenacanthus hybodoides, Ag., do not belong to Cla- 

 dodus mirabilis, Ag., as has been asserted, but are allied to 

 Hyhodus in their narrow, compressed non-expanded bases. 

 Mr J. W. Barkas long ago expressed his opinion that most 

 of these so-called Cladodi are in reality " Hybodi " (M. Eev, 

 " Dent. Surgery," Feb. 1874), though he seems to think that 

 the great difference between Cladodus and Hyhodus lies in 

 the former having the outermost denticles larger than the 

 intermediate ones, and consequently admits some of these 

 Coal-measure specimens to the genus Cladodus. 



Ctenacanthus hyhodoides has, therefore, nothing to do with 

 Cladodus ; and as regards the other species, I rather think 

 that if we knew the creatures to which they belonged, that 

 they would turn out to represent several types possibly very 

 different from each other. But of this I have now no doubt, 

 that the Cladodontidse, whether they had spines or not, or 

 whatever the shape of their spines, if they had any, constitute 

 a very different family from the Hybodontidae, and that they 

 were a more archaic group ; while the Hybodonts, on the 

 other hand, were closely allied to the Cestraciontidse. 



For if Tristychius be a Hybodont, we have now some clue 

 to the structure of an ancient representative of the family. 



Tristychius, Agassiz. 



A specimen of Tristychius, from Eskdale, allied to, if not 

 identical with, Agassiz's T. arcuatus, shows the greater part 

 of the body with the head, the pectoral fins, and two dorsal 

 fins. Each of the dorsal fins has a spine in front. The pectoral 

 shows two large basal pieces, which I interpret as mesoptery- 

 gium and metapterygium ; the propterygium being either 

 small or fused with the mesopterygium, as in Cestracion, while 

 there is no trace of the segmented prolongation of the me- 

 tapterygium which we saw in the E. Kilbride Cladodus. 



This interesting specimen is fatal to Mr T. Stock's idea 

 that the spines in this genus were paired,^ as well as to its 



1 Ann. and Mag. Nat. Hist. (5) XIL, 1883, p. 188. 



