295 

 The measurements of the fossil are as follows: 



Lines. 



Depth of bone 18-20 



Breadth of the bone . - 17 



Length of the larger tooth 8 



Width of same 8 



Thickness of same - 5| 



Depth of enamel 6| 



Length of smaller tooth 7f 



Width of same -■ 1\ 



Thickness of same x -. 4£ 



Depth of enamel 6£ 



I have arranged Hadrodus with the Pycnodonts, though, like Placodus, the 

 discovery of additional material may prove it to be a sauropterygian reptile. 

 Of Placodus, Professor Owen remarks that the "teeth are implanted by 

 short simple bases in distinct hollow sockets," (Palaeontology, 218;) and Meyer 

 says, "In wircklichen Alveolen stecken eigentlichen nur die Sclmeidezahne 

 mit gut ausgebildeten Wurzeln, der Wurzeltheil der vibrigen Zahne ist mehr 

 mit dem Knochen, dem die Zahne angehoren, verbunden." Hadrodus in 

 the relation of the teeth would appear to be different, as they are firmly 

 co-ossified, by short bases with the border of the jaw. They exhibit no 

 trace of implantation by sockets, though the successional teeth before being 

 established in a fixed manner in functional position must appear at least to 

 spring from sockets. 



ELASMOBRANCHU. 



Order Plagiostomi. 



PTYCHODUS. 



Ptychodus Mortoni. 



The extinct genus of cestraciont fishes above named was inferred by 

 Agassiz, from isolated teeth, the only parts yet found which can be with any 

 certainty referred to the same animal. A number of species have been indi- 

 cated, mostly by the same authority, from specimens found in the Cretaceous 

 formations of Europe and America. 



Teeth of Ptychodus Mortoni have been discovered in the Cretaceous de- 

 posits of Alabama, Mississippi, and Kansas, but I have seen none from the 

 corresponding formation of New Jersey or elsewhere. 



The Smithsonian Institution has submitted to my examination a collection 



