226 



FOSSIL FISHES OF THE ENGLISH CHALK. 



Type Species. — Ptychodus mammillaris, from the English Chalk. 



Remarks. — This is an exclusively Upper Cretaceous genus, of which most 

 of the species are known only by scattered teeth. The plan of arrangement of 

 the dentition has been discovered in P. decurrens (PI. LI, figs. 4-0, 9-12; text- 

 figs. 70, 71, 76, 77), and in P. mortoni; while the jaw-cartilages and vertebral 

 centra have hitherto been observed only in the former species. 



The number of teeth in each jaw is very great, and the new teeth from behind 

 increase only slowly in size. Williston 1 has counted about 550 teeth in a well- 

 preserved upper dentition of P. mortoni, where they are arranged in seventeen 

 antero-posterior rows; and he estimates that the total number cannot have been 



Fig. 70. Ptychodus decurrens, Agassiz; remains of jaws with dentition in small block of chalk, slightly reduced 

 in size. — Zone of Holaster subglobosus ; Glynde, Sussex. Willett Collection, Brighton Museum. A. Upper 

 aspect, showing extent of decayed symphysis ; B. lower aspect, without symphysis ; C. part of upper 

 dentition, oral aspect ; D. posterior end-view of middle upper teeth ; E, F. posterior and anterior teeth 

 respectively of the left upper inner paired row. md., mandible ; piq., pieces of cartilage of upper jaw 

 (pterygo-quadrate) ; o'., i'., teeth of upper middle and inner paired rows ; o., I., teeth of lower middle and 

 inner paired rows. From Quart. Journ. Geol. Soc, vol. lx (1904). 



less than 600. They are fewer in P. decurrens, where there is no evidence of more 

 than thirteen or fifteen rows. In two specimens referable to the latter species 

 (Text-figs. 70, 76), some teeth of both jaws are actually seen in their natural 

 position; Avhile in one of the specimens (Text-fig. 70), the dentition is shown to 

 be restricted to the long symphysial portion of the jaw, as represented in the 

 restored sketch, Text-fig. 71. The rami of the jaw meet in an acute angle at 

 their elongated symphysis, and the cartilages resemble those of existing Elasmo- 

 branchs in being only superficially calcified. 



1 S. W. Willistou, "Cretaceous Fishes," Univ. Geol. Surv. Kansas, vol. vi (1900), p. 239, 

 jils. xxv-xxvii. 



