284 CRETACEOUS LAMELLIBRAXCHIA. 



passes from the umbo to near the postero-ventral margin. The shell is ornamentetl 

 with small concentric ribs, which bend dorsally in front of the furrow and become 

 parallel to the anterior margin. 



Measurements : 



(1) (2) 



Length . . . 15 . S mm. 



Height . . . 11 . 0-5 ., 



Thickness . . . — . (rb ,, 



Affinities. — The very slight curvature of the anterior margin and nf the anterior 

 ribs distinguishes this species from the one described below. 



Remarlcs. — T. DaUasi occurs commonly boring in wood. 



Ti/2w. — From Potton ; in the Sedgwick Museum. 



Dintrihufion. — Lower Greensand of Potton. 



TuRNUS, sp. Plate XXXVIII, figs. KJ, 17. 



Descrijdiun. — Shell oval, inflated, posterior end wedge-like. Ventral margin 

 curved ; posterior mai'gin 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 i;mbo 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 I'ibs I)end forwards. 



Measurements ; 



(1) (2) 



Length . . . 14 . 13 mm. 



Height . . . 11 . 9-5„ 



Thickness . . . 12 . 10 „ 



(1—2) Gault, Folkestone. 



Affinities. — This species resembles T. argoiinensis (Buvignier),^ but exact 

 comparison is difficult since only a small internal cast is figured by Buvignier. 

 The English form appears to differ from T. argonneasis 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. 



Disfributioii. — Lower Gault of Folkestone. 



1 ' Statist, ycul. iiiiu. ot jiak-out. de la Meuse,' Atlas (1852), p. 6, pi. vi, figs. 33—39. 



