MYLIOIiATID.^J. 
143 
Ptychodus polygyrus, Agassiz. 
1822. Teeth allied to Diodon, G. A. Maiitell, Foss. South Downs, 
p. 231, pi. xxxii. figs. 23, 24. 
1837. Ptychodus polygyrus, W. Buckland, Geol. & Min. 2nd edit, 
vol. ii. pi. xxvii.y. 
1839. Ptychodus polygyrm, L. Agassiz, Poiss. Foss. vol. iii. p. 156, 
pi. XXV. figs. 4-11, pi. XXV. b, figs. 21-23. 
1843. l^ychodus iatissimus, L. Agassiz, tmn. cit. p. 157, pi. xxv. a. fig. 8. 
1860. Ptychodus polygyrus, 11. \V. Gibbes, Journ. Acad. Nat. Sci. 
Philad. [2] vol. i. p. 299, pi. .xlii. figs. 6, 6. 
1850. Ptychodus jiolygynis, F. Dixon, Foss. Suss. p. 363, pi. xxx. 
fig. 9, pi. xxxi. fig. 10. 
1860. Ptychodus latissitmts, F. Dixon, op. cit. p. xii, pi. xxx. figs. 1, 2. 
1852. Ptychodus polygyrvs, V. Kiprijanofi) Bull. Soc. Imp. Nat. Moscou, 
vol. xxv. pt. ii. p. 494, pi. xiii. fig. 6. 
1803. Ptychodus polygyrus, S. J. Maclde, Geologist, vol. vi. p. ICl, 
pi. ix. 
1868. IHychodus polygyrus, .1. Leidy, Proc. Acad. Nat. Sci. Philad. 
p. 208. 
1887. I'tychodus polygynis, A. S. Woodward, Quart. Journ. Geol. Soc. 
vol. xliii. p. 127, pi. x. fig. 11. 
Typie. Detached teeth ; British Museum (in jiart). 
The median transverse ridges upon the dental crown are relatively 
large, and curve round more or less at the lateral extremities, being 
sometimes completely reflexed. The finely marked marginal area 
is usually of considerable extent and granulated, showing only a 
tendency towards concentric rugosity immediately adjoining the 
extremities of the median transverse ridges. 
Among the specimens at first referred to this species, Agassiz 
distinguished four types, which he considered might possibly prove 
to be specifically distinct. In the absence of more satisfactory 
materials, however, it is stiU advisable to retain the original 
arrangement. 
Form. Loc. Senonian and Turonian : 8.E. England, France, 
Belgium, Eussiii. Upper Cretaceous : United States, America. 
Unless otherwise stated, the following specimens were obtained 
from undetermined horizons in the Upper or Lower Chalk : — 
4385, 4408, 4413. Three of the type specimens figured by Agassiz, 
tom. cit. pi. xxv. figs. 4, 5, 8 ; Lewes. Mantell Coll. 
4452. Plaster cast of lower tooth, referred to P. Iatissimus by 
Agassiz, tom. cil. p. 1,57, pi. xxv. a. fig. 8 ; Sussex (?). 
Mantell Coll. 
