166 Lr. W. J, Barkas — Hyhodus, a Coal-measure Fish. 



c'est-a-dire que la cone secondaire externe est sensiblement plus 

 developp^ que les autres." 



According to Agassiz, then, tlie only external differences between 

 JJyhodus and Cladodus are, that in the former the secondary denticles 

 decrease in size as they proceed from the centre, while in the latter 

 they increase ; and also that in Hyhodus the secondary denticles are 

 unequal in number on each side of the principal cone, while in 

 Cladodus they are equal on each side of the centre denticle. 



With regard to the microscopical structure of these teeth, the 

 above is the only author that I am acquainted with who has described 

 it, and he says, " Les dents du genre Cladodus montrent en g6n6ral 

 la meme structure fondamentale que celles des Hijhodes ; " but he 

 considers that in the former teeth the medullary canals are larger 

 and branch more seldom, especially in the crown, and that the calci- 

 gerous tubes are large, very short, and branch freely. The internal 

 differences are, therefore, very slight, if there be any at all ; for I 

 cannot understand how he arrived at his conclusions about the canals 

 and tubules. I have examined numerous teeth of all kinds under 

 the microscope, and I should not like to say positively with regard 

 to any tooth that it was a different genus merely because the medullary 

 canals were larger and branched less than in another specimen 

 closely allied to it in external appearance, for in no other character- 

 istic do they vary so much ; even the teeth of the same species are 

 different in this respect. 



We have now seen what are the chief differences between these 

 teeth, and what is the geological position which up to this time 

 they have been considered to hold. It only remains for me now to 

 compare the specimens I have under my notice at present with 

 drawings of acknowledged Hybodus and Cladodus from the " Poissons 

 Fossiles" and "The Geological Survey of Illinois." 



Teeth of Hyhodus (Figs, 1 and 2), compared with teeth of Cladodus (Figs. 3-6). 

 [Since the above paper was written I have examined the minute 

 structure of the teeth of Hyhodus obtained from the Wealden of 

 Sussex and from the Northumbrian Coal Measures, and I find that 

 they are almost identical. Engravings, exhibiting their micro- 

 scopical structures, appeared in the " Monthly Eeview of Dental 

 Surgery" for February, 1874.] 



