48 CRETACEOUS LAMELLIBRANCHIA. 
anterior margin rounded ; posterior nearly straight and very oblique, forming an 
acute angle with the ventral margin. Umbones rather prominent, with a carina 
which extends to the postero-ventral angle, and cuts off a depressed postero- 
dorsal area. Ornamentation consists of very fine radial ribs. Hinge a little 
curved; teeth numerous, gradually becoming larger and more oblique toward 
the extremities. 
Measurements : 
() (2) (3) (4 (5) (6) 
Length : ‘ ; : . 58 50 37 57 62 48mm. 
Height ; : : : PPADS TSC 20 1399439 35), 
Thickness. : 48 35 — 39 39 30 ,, 
(1) From the Chloritie Marl, Warminster. (2) From the base of the Chalk Marl, Chard. 
(3) From the Upper Greensand, Kingskerswell, (4—6) From the Cenomanian, Rouen. 
Affinities.—This species is very similar to U'rigonoarca ligeriensis (d’Orbigny),’ 
with which it may prove to be identical; that form is said to be distinguished by 
its greater convexity and more widely separated umbones. 
Jt appears also to be related to d’Orbigny’s Arca royana,’ but that form is 
only known by its cast. 
Remarks.—This species was founded on internal casts from the Cenomanian 
of Rouen. The only specimen I have seen with the shell preserved is a silicified 
example from the Greensand of Kingskerswell (British Museum, No. L. 1853); an 
internal mould from the same locality and collection shows the character of the 
hinge. The internal casts from various localities agree perfectly with specimens 
obtained from Rouen. 
Types.—Some specimens from Rouen, but apparently not the type, are in the 
d’Orbigny Collection. 
Distribution.—Base of the Lower Chalk (zone of Schlenbachia varians) of 
Chard. Chloritie Marl of Maiden Bradley, Urchfont (near Devizes) and War- 
minster. Upper Greensand of Kingskerswell. 
1 «Pal. Frang. Terr. Crét.,’ vol. iii (1844), p. 227, pl. ceexvii, figs. 1—3 (not 4, 5). 
2 Tbid., p. 243, pl. ecexxvii, figs. 3, 4. 
