236 FOSSIL FISHES OF THE ENGLISH CHALK. 



1873. Ptychodus laLissimus, O. Eees, Proc. G-eol. Assoc, vol. iii, p. 117. 



1887-89. Ptychodus latissimus, A. S. Woodward, Quart. Journ. Geol. Soc, vol. xliii, p. 127, pi. x, 



fig. 12 ; and Catal. Foss. Fishes B. M., pt, i, p. 147. 

 1889. Ptychodus polygyrus, A. S. Woodward (errore), Catal. Foss. Fishes B. M., pt. i, pi. v, tig. 7. 

 1902. Ptychodus latissimus, M. Leriehe, Ann. Soc. Geol. Nord, vol. xxxi, p. 91, pi. ii, tigs. 1 — 7. 

 1906. Ptychodus latissimus, M. Leriehe, Mum. Soc. Grc'ol. Nord, vol. v, pp. 66, 73, pi. v. 

 1911. Ptychodus latissimus, G. E. Dibley, Quart. Journ. Geol. Soc, vol. lxvii, p. 270, pi. xx, figs 3 — 5. 

 1911. Ptychodus dixoni, G. E. Dibley, loc. cit., vol. lxvii, p. 270, pi. xx, figs. 1, 2. 



Type. — Detached teeth ; British Museum. 



Specific Characters. — A large species, of which the lower median teeth some- 

 times measure G cm. in transverse diameter. Median portion of tooth flattened 

 or gently raised and crossed by very coarse transverse ridges, with little or no 



Fig. 74. Ptijchodus latissimus, Agussiz ; teeth of lower jaw, about one half nat. size. — Zone of Terebratulina 

 gracilis; Conde (Nord), France. Geological Museum, University of Lille. After M. Leriehe. 



curvature at the lateral ends and never forming loops. Marginal area usually 

 narrow, but often wide in the lower median teeth ; more or less coarsely granu- 

 lated, the granules sometimes in curved rows continuing the ends of the transverse 

 ridges, but never crossed by radiating grooves. Transverse ridges of lower 

 median teeth from 6 to 8 in number. 



Description of Specimens. — The type specimens of this species in the Mantell 

 Collection include three isolated lower median teeth, of which the least-coarsely 

 marked is re-drawn in PI. L, fig. 1. It is much wider than long, and its gently 

 raised median portion is crossed by seven complete transverse ridges, of which the 

 hinder three are sinuous in the middle and curve at their lateral ends into rows of 

 granulations on the wide marginal area, while the anterior three ridges nearly 

 converge into a gyration. The whole of the marginal granulation is coarse. 



