Davis — On the Fossil Fish of the Cretaceous Formations of Scandinavia. 375 



Genus. Ptychodus. Agassiz. " Poiss. Foss.," vol. iii., p. 150. 1839. 

 Syn.—kulodMS, F. Dixon, " Foss. Sussex.," p. 366. 1850. 



Teetli quadrate in form, with elevated crown, somewhat overhanging, and 

 sharply separated from the root by a constriction. The crown is enamelled, 

 and ornamented with large transverse or radiating ridges, surrounded by a more 

 finely-marked marginal area of greater or less extent. The surface of attachment 

 of root is smooth. In one jaw, presumably the lower, the median series of teeth 

 is the largest, and the lateral rows are arranged symmetrically, diminishing in 

 size to the left and right. In the opposing dentition the median series is very 

 small, and the first lateral row on each side large, with the outer lateral series 

 successively diminishing in size. The vertebrae are very deep compared with 

 their length, complete, and apparently cyclospondylic in structure. 



The well-known teeth of Ptychoduswere supposed by Mantell * to have formed 

 the dental armature of a teleostean fish nearly allied with the Diodon. The 

 observations of Agassiz,t and the microscopical investigations of Owen, J led to 

 -the conclusion that the arrangement of the teeth of Ptychodus closely resembled 

 that of the Cestraciont sharks. The absence of any specimens showing the teeth 

 in actual position has led to a general acceptance of the view enunciated by 

 Agassiz. More recently, Cope,§ in America, and Smith Woodward, || in this 

 country, have been able, by the discovery of more perfect specimens, to arrive at 

 conclusions of great importance. Cope demonstrated that the spinous processes 

 supposed to have been the dorsal spines of Ptychodus were the fin-rays of a 

 Teleostean fish ; and Woodward has shown that the teeth have no agreement with 

 the dental arrangement in the Cestraciont shark, but that the dentition is that of a 

 true ray. The arrangement of the teeth is in parallel rows, crossing the rami at 

 right angles. There is a median row in each jaw, and on either side of it there 

 are series placed symmetrically right and left. In the upper jaw the median teeth 

 are small ; the largest are placed in the first lateral series, from which there is a 

 gradual diminution in size outwards. In the lower jaw the median teeth form 



* Fossils of the South Downs, p. 231. 1822. 



t Eecherclies sur les Poissons rossiles, vol. iii., pp. 66-59, 150-158, 162. 



X On the Structure of Teeth, Brit. Assoc. Eep., 1838, Trans. Sect., p. 140; and Odontography, 

 pp. 57-59, pis. snn., six. 



§ Yertebrata of the Cretaceous Pormations of the West (U. S. Geol. Surv. Terr., 1875), 

 p. 244 A-F. 



II Quart. Joum. Geol. Soc, vol. xUii., pp. 123-130., pi. x. 



