418 Proceedings of the Boyal Physical Society. 



of Psephodus and Cochlioclus are generically indistinguishable 

 from those of Helodus. 



As seen in Psephodus, which is one of the least specialised 

 of the Cochliodontidse, the posterior teeth lose their deep 

 roots, become flattened, and tend to fuse together into broad 

 inrolled plates. I have a specimen of the broad tooth plate 

 of Psephodus magnus, Ag., which, by a groove, is divided 

 longitudinally into two portions, which pretty closely repre- 

 sent not uncommon forms of Relodus planus. The grooves 

 on Pcecilodus, Deltodus, etc., also, to my mind, represent the 

 morphological origin of those plates, from the fusion of 

 smaller and narrower separate teeth. Pleurodus is a well- 

 known form in which each plate is due to the union, back to 

 front, of a row of helodont teeth, whose lateral extremities 

 still tend to project free on each side. 



That the cochliodonts all possessed dorsal spines seems 

 highly probable. Those of Pleurodus have been described 

 by Hancock and Atthey. 



Petalodontid^. 



If we take Ctenoptychius apicalis, Ag., as the type of its 

 genus, I must own that I fail to see any valid reason for 

 separating Gtenopetalushova it ; and even Petalodus is scarcely 

 entitled to distinction. Rarpacodus differs in having only 

 one fold or plait at the junction of the crown and root ; and 

 it is in this genus that Mr J. Davis proposes to include 

 Ctenoptychius pectinatus of Agassiz. But Ctenop>tychius pec- 

 tinatus is not provided with any " fold " of enamel below the 

 crown comparable to those in Ct. apicalis, or to the single one 

 in Rarpacodus; while its root differs very considerably in 

 shape, being divided below into a number of small rootlets, 

 somewhat after the manner of Polyrhizodus. A new genus 

 is, therefore, necessary for it, for which I propose the name 

 Callopristodus. 



Oracanthus. 



Some time ago Mr E. Craig of Langsyde, Beith, lent me a 

 small spine from the shale above the 9-inch coal at Broad- 



