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 : 









(1) (2) (3) (4j (5) 



(6) 



Length .... 



. 58 50 37 57 62 



48 mm. 



Height .... 



. 45 37 27 39 39 



33 „ 



Thickness .... 



. 48 35 — 39 39 



30 „ 



(1) From the Chloritic Marl, Warminster. 



(2) From the base of the Chalk Marl, Chard. 



(3) Prom the Upper Greensaud, Kingskerswell, 



(4 — 6) From the Ceuomanian, 



Rouen. 



Affinities. — This species is very similar to Trigonoarca ligeriensis (d'Orbigny), 1 

 with which it may prove to be identical ; that form is said to be distinguished by 

 its greater convexity and more widely separated umbones. 



It appears also to be related to d'Orbigny' s Area royana," but that form is 

 only known by its cast. 



Remarks. — This species was founded on internal casts from the Ceuomanian 

 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 Scliloenbachia varians) of 

 Chard. Chloritic Marl of Maiden Bradley, Urchfont (near Devizes) and War- 

 minster. Upper Greensand of Kingskerswell. 



1 ' Pal. Franc. Terr. Cret.,' vol. iii (1844), p. 227, pi. cccxvii, figs. 1—3 (not 4, 5). 



2 Ibid., p. 243, pi. cccxxvii, figs. 3, 4. 



