MYLI0BATID2E. 
139 
1856. Ptychodus polygyrus, C. E. Fischer, Allgem. deutsche naturh. 
Zeit. Dresden, n. s. vol. ii. p. 140, figs. 31-33. 
1872. Ptychoclus decurrens, H. E. Sauvage, Biblioth. Ecole Hautes 
Etudes, vol. v. art. 9, p. 18. 
1875. Ptychodus decurrens, H. B. Geinitz, Palseontographica, yoI. xx. 
pt. i. p. 296, pi. lxiv. figs. 24, 25. 
1878. Ptychodus latissimus, St, Zarecznego, Sprawozdanie Komisyi 
Fizyjograf. Galicyi, vol. xii. p. 200, pi. yiii. fig. 8. 
1878. Ptychodus decurrens, A. Fritsch, Kept. u. Fische bolim. Kreide- 
form. p. 14, fig. 34. 
1882. Ptychodus decurrens, F. A. Quenstedt. Handb. Petrefakt. 3rd edit, 
p. 281, woodc. fig. 86, pi. xxi. figs. 63, 64. 
1887. Ptychodus decurrens, A. S. Woodward, Quart. Journ. Geol. 
Soc. vol. xliii. pp. 123-130, pi. x. figs. 1-10, 13. 
Type. Detached teeth ; one in British Museum."" 
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, and at either extre¬ 
mity pass insensibly into the finely marked marginal area, with 
little or no terminal reflexion ; sometimes those near the anterior 
border are irregularly branching and broken, and approximate 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. cit., the lower median teeth are remarkably 
high, and both these and the first lateral series are much trans¬ 
versely elongated. Id 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. 6f Loc. Senonian and Turonian : S.E. England and France. -Sc 
Turonian and Cenomanian : Germany and Bohemia. 
Except when otherwise stated, the following specimens were ob¬ 
tained from undetermined horizons in the Upper or Lower Chalk :— 
P. 5449. Lower median tooth, figured among the type-specimens of 
P. decurrens by Agassiz, tom. cit. pl. xxv. b. fig. 8 ; Lewes, 
Sussex. Mantell Coll. 
25773-4, 25776. Three varieties of lower median teeth, figured by 
Dixon, op. cit. pi. xxx. figs. 7, 8, pi. xxxi. fig. 1; Brighton, 
Sussex. Duron Coll. 
