II yliokatid.t:. 
139 
18o0. Ptychodus polyyyrvf, C. K. Fisclier, Allgein. deutsclie uaturh. 
Zeit. Dresden, n. s. vol. ii. p. 140, figs. 3]-4i:l. 
1872. Ptychodua dectirnms, H. E. .Sauvage, Iliblioth. Ecole Hautes 
Etudes, vol. V. art. 9, p. 18. 
1875. Ptychidus deci/irens, 11. I!. Goinitz, Palieontographica, vol. xx. 
pt. i. p. 2SX5, pi. Ixiv. figs. 24, 26. 
1878. Ptychodua latksimua, 8t. Zarecznego, Sprawozdauie Komisyi 
Fizyjogi’af. Galicyi, vol. xii. p. 200, pi. viii. fig. 8. 
1878. Ptychodus decurrens, A. Fritscli, Kept. u. Fische bolim. Kreide- 
form. p. 14, fig. 34. 
1882. Ptychodus decurrens, F. A. Quenstedt. llandb. Petvefakt. 3rd edit, 
p. 281, woodc. fig. 80, pi. xxi. figs. ()3, 04. 
1887. Ptychodus decurrem, A. S. Woodward, Quart. .Journ. Geol. 
Soc. vol. xliii. pp. 123-130, pi. x. figs. 1-10, 13. 
Type. Detached teeth ; one in British JIusoum. 
The median lower teeth are moderately elevated, the coronal 
contour being greatly arched ; and the lateral teeth, both upper and 
lower, are only slightly raised. The median transverse ridges of 
each tooth are straight, small, and numerous, aud at either extre- 
mity pass insensiidy into the finely marked marginal area, with 
little or no terminal reflexion ; sometimes those near the anterior 
border are irregularly branching and broken, and aj^proximate to a 
diagonal direction. 
The arrangement of the complete dentition of this species is 
known, and two types are distinguishable. In the one described by 
the present writer, loc. ciL, the lower median teeth ai'e remarkably 
high, and both these and the first lateral series are much trans- 
versely elongated. In the second type, made known both by Dixon 
and the present writer, the lower median teeth are less elevated, 
and both these and the first lateral series are considerably longer in 
proportion to their breadth. The latter dentition may belong to a 
distinct species {P. depressus, Dixon), but, upon present evidence, 
it can only be regarded as a variety. 
Form. Loc. Senonian and Turonian : S.E. England and France. 
Txironian and Cenomanian : Germany and Bohemia. 
E.xcept when otherwise stated, the following specimens were ob- 
tained from undetermined horizons in the Upper or Lower Chalk; — 
P. 5449. Lo-wer median tooth, figured among the type-specimens of 
P. decurrens by Agassiz, tom. eit. pl. xxv. h. fig. 8 ; Lewes, 
Sussex. Mantell Coll. 
25773-4, 25776. Three varieties of lower median teeth, figured by 
Dixon, op. eit. pi. xxx. figs. 7, 8, pi. xxxi. fig. 1; Brighton, 
Sussex. Dixon Coll. 
