MALAC0Z0A. TROPIOPODA. LAMELLIBRANCHIATA. 261. 



Crassina convexiuscula. Brown, Illustr. PI. 18. f. 7? — Cyprina 

 compressa. Turt Brit. Biv. 136. PI. 1 1. f. 22,23?— Crassina multi- 

 costata. Smith, Wern. Mem. viii. 104. PI. 1. f. 20. 



<6. Astdrte compressa. Compressed Astarte. 



Shell ovato- trigonal, compressed ; with the anterior end 

 shorter, the umbones pointed and approximated; the dorsal 

 slope straight for half its length, then convex, with a linear- 

 oblong, obliquely striated impression, of which the sides slope 

 inwards; the anterior slope nearly straight to the end of the 

 oblong impression ; the valves moderately thick, with irregular 

 flattened concentric ridges and shallow sulci, obliterated toward 

 the margins ; the muscular impressions very large, the anterior 

 obovato-lunate ; the margin plain, with a broad flat space be- 

 tween it and the pallial impression ; the epidermis yellowish- 

 brown. Length an inch and a-half, height an inch and a- 

 quarter. 



This species ccmes nearest in form to Astarte elliptica, but 

 has little affinity to Astarte multicostata, which has by several 

 authors been strangely confounded with it. No two species 

 could be more distinct than one which is ovato-trigonal, with 

 obsolete irregular ridges and sulci, and one rotundato-trigonal 

 with most regular, small, very numerous well-marked ridges 

 and sulci. Dr. Fleming's Astarte compressa is the same as 

 this species, in so far as the " dead valves from St. Andrew's 

 Bay upwards of an inch and three-twelfths in length," are 

 concerned, as I have compared them with an Aberdeen speci- 

 men. The description and figure of Montagu agree with the 

 species here described. Brown's Crassina sulcata, (PI. 18. 

 f. 10.) cannot be Montagu's Venus sulcata, the margin in the 

 former being crenate, in the latter plain. 



Venus compressa. Mont. Test Brit. SuppL 43. PI. 26. f. 1, I. — 

 Astarte compressa. Flem. Brit Anim. 440. 



Genus 4. Cytherea. 



Shell equivalve, inequilateral, moderately convex, sub- 

 orbicular, concentrically striated. Umbones small, ap- 

 proximated, curved forwards. Frontal slope concave, 

 with a distinct cordate depression. Hinge strong ; the 

 right valve with three divergent teeth, and three depres- 

 sions, and an anterior elongated obtuse ridge, succeeded 

 by a marginal groove; left valve with four divergent 



