300 



Ptychodus Whippleyi. 



Marcon, in his Geology of North America, describes and figures a tooth 

 from the Cretaceous formation near Galisteo, New Mexico, and refers it to a 

 peculiar species under the above name. 



A similar tooth submitted to my inspection by Dr. Benjamin F. Slnimard, 

 from the Cretaceous rocks of Texas, is represented in Figs. 19, 20, Plate 

 XVIII. It is remarkable for the abrupt nipple-like prolongation of the 

 crown. 



The tooth is unsymmetrical, and probably held a lateral position in the 

 series. The base of the crown is quadrate, witli the fore and outer borders 

 forming a single curve, while the other borders form a nearly right angle. 

 The nipple-like eminence of the crown inclines, as I suppose, outwardly. 

 The posterior sinus is shallow. The rugae of the surface of the crown cross 

 the summit transversely and diverge and branch descending upon the sides of 

 the cone. They are comparatively feeble, but this condition may be partially 

 due to friction. The surface of the base of the crown appears rather nodu- 

 lated than reticulated. 



The breadth of the tooth at the base of the crown is 7 lines transversely 

 and fore and aft ; its height from the bottom of the root 8 lines. 



The tooth resembles that of Ptychodus altior of Agassiz, from the chalk of 

 Sussex, England, as represented in Fig. 10, Plate XXX, of Dixon's Geology 

 of Sussex. 



ACRODUS. 



This extinct genus of cestraciont sharks, first described by Agassiz, was 

 represented in Europe l)y many species whose remains occur in the various 

 formations from the Permian to the Cretaceous inclusive. 



ACEODUS HUMILIS. 



A specicss to which this name has l)een given is indicated by an isolated 



