234, CRETACEOUS LAMELLIBRANCHIA. 
passes from the umbo to near the postero-ventral margin. The shell is ornamented 
with small concentric ribs, which bend dorsally in front of the furrow and become 
parallel to the anterior margin. 
Measurements : 
(1) (2) 
Length 4 - 15 : So) mm 
Height : Ach OO, 
Thickness , — : 65 
29 
Affinities—The very sight curvature of the anterior margin and of the anterior 
ribs distinguishes this species from the one described below. 
Remarks.—'l'. Dallasi occurs commonly boring in wood. 
Type.—From Potton; in the Sedgwick Museum. 
Distribution.—Lower Greensand of Potton. 
Turnus, sp. Plate XXXVIII, figs. 16, 17. 
Description.—Shell oval, inflated, posterior end wedge-hke. Ventral margin 
curved ; posterior margin rounded. Antero-ventral excavation angular. Umbones 
prominent, anterior, incurved, close together. An external furrow and a correspond- 
ing internal rib extend from the umbo to the ventral margin. A strong internal 
rib passes from the umbo to near the postero-ventral edge. 
Posterior to the dorso-ventral groove the shell is ornamented with narrow, 
regular concentric ribs; for a short distance in front of the groove the ribs bend 
sharply upwards, and soon become smaller and close together ; on the dorsal part 
of the anterior end the ribs bend forwards. 
Measurements : 
(1) (2) 
Length : 14 15 mm. 
Height 2 : : IE ; OP) 
Thickness , : ; 12 ; OP "5 
(1—2) Gault, Folkestone. 
Affinities —This species resembles 7’. argoniensis (Buvignier),! but exact 
comparison is difficult since only a small internal cast is figured by Buvignier. 
The English form appears to differ from 7’. argonnensis in the larger size and 
more angular character of the anterior gape, and in the smaller and closer ribs, 
especially on the anterior part of the shell. 
Distribution—Lower Gault of Folkestone. 
1 «Statist. géol. min. et paléont. de la Meuse,’ Atlas (1852), p. 6, pl. vi, figs. 33—39. 
