CESTRACIONTS. 61 



Ptychodus, differs from that of Acrodus in the greater number and 

 more parallel course of the medullary canals, their fewer branches, 

 and in the absence of an external layer of fine parallel tubes. 



The Psammodontoid teeth which are elongated, more or less 

 contracted and truncated at the two extremities, and of which the 

 surface of the crown is reticulated, are now regarded by M. Agassiz 

 as indicating a distinct subgenus, to which he gives the name of 

 Strophodus, (PI. 17, fig. 4). Other fossil teeth, which resemble in 

 general structure those of Psammodus, but which have the centre of 

 the crown elevated into an obtuse transverse cone, and traversed by a 

 ridge, from which oblique furrows diverge, but with a more or less 

 transverse direction, towards the circumference and there ramify, 

 have been referred by the same authority to a genus called Orodus. 

 These fossils occur in the older secondary rocks. 



Similar fossil teeth, having the crown in the form of an elevated 

 obtuse cone, but perfectly smooth on the surface, form the type of the 

 genus Helodus. Agas. 



In a fourth form, the crown is compressed and elevated, and 

 sometimes terminates in a sharp edge like the tooth of a Carcharias ; 

 its base is always surrounded by a series of concentric folds. This 

 is the type of the genus Chomatodus. Agas. 



A fifth form of Psammodontoid teeth, in which the crown is 

 raised, subcompressed and subdivided by deep transverse ridges into 

 dentations, varying as to number and degree of sharpness or obtuse- 

 ness, has given rise to the establishment of the genus Ctenoptychius, 

 Agas. 



23. Petalodus. — Intheteeth of many of the suhgenersLoi Psammodus, 

 the crown is produced into a median or submedian ridge ; if the body 

 of the tooth be supposed to be still more compressed, so that the 

 ridge should terminate the contour of the crown like a trenchant 

 edge, there will then be produced the lamelliform figure which cha- 

 racterizes the teeth of the subgenus in question. 



In the specimen of the tooth of Petalodus Hastingsii now before me, 

 which I owe to the kindness of Sir Philip Egerton, the trenchant margin 

 is slightly convex and finely serrated, the crown of the tooth is invested with 

 a thin layer of dense enamel, with a smooth and shining surface, the 



