LAMELLIBRANCHI ATA. Ill 



Plicatula EQUICOSTATA, n. Sp. 

 (Plate v., fig. 5, a, b, c.) 



Shell obliquely ovate, plano-convex, very inequivalvular and 

 inequilateral ; covered with numerous equal-sized ribs. Upper 

 valve flat or slightly concave, marked with concentric lines of 

 growth, which may be raised into lamellae especially towards the 

 margin, and ornamented with 15 — 20 radiating ribs. These ribs 

 are broad and slightly arched ; they are separated by only a narrow 

 groove. There are indications that spines may be developed in 

 some specimens at points where the lines of growth cross the ribs. 

 Towards the margin of the shell additional and sharper ribs may 

 be added. 



The lower valve is deep and gibbous, attached by a small area 

 at its umbo. It is provided with irregular imbricating growth- 

 lamellae and zoned by a few, faint or strong varices. These 

 structures are covered with numerous ribs (about 50) which 

 are rounded and gently arched, separated by narrow and simple 

 interspaces crossed by lines of growth. 



In one specimen, whose shell has been partly denuded, the 

 ribs on the small valve appear to have been much more numerous 

 than in our type. 



Affinities and differences. This species approaches most nearly 

 to P. placunea (Lamk.) and P. imbricata (Koch.). From the former 

 it is distinguished by its gibbosity and the equality of its ribs, and 

 from the latter by its flat or concave upper valve. 



Locality. Upware. 



Lima Tombeckiana, d'Orbigny. 



L. semisulcata, Forbes, 1845. Quart. Jour. Geol. Soc. p. 248 



(non Nilsson). 

 L. Tombeckiana, d'Orbigny, 1845. Pal. Fr. Terr. Cre't. III. p. 



534, pi. 415, f. 13—17. 

 „ „ Pictet and Campiche, Pal. Suisse, Vol. v., Terr. 



Cret. St Croix, p. 148. 



Distinguished from L. Bupiniana by its broader, closer-set ribs. 



