HALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 263 



3. Cytherea undata. Waved Cytheraa. 



Shell suborbicular, convex, thin, subdiaphanous, with nu- 

 merous concentric strioe, and larger rugae, which are more or 

 less sinuous, as is the very thin inferior margin of the valves ; 

 no distinct impression before the umbones ; two middle cardi- 

 nal teeth of the last valve prominent and thin ; three teeth in 

 the tight valve, the middle tooth larger and cleft; the exter- 

 nal surface glossy ; the internal smooth, with the submarginal 

 space glossy, and faintly marked with marginal striae ; the 

 muscular impressions oval, the pallial invaded by a deep ob- 

 tuse sinus running upwards beyond the middle of the shell ; 

 the exterior white, or yellowish or reddish-white, sometimes 

 ierruginous toward the umbones, the anterior of a paler red- 

 dish-white. Length an inch and a-half, height about the 

 same. 



Not very uncommon in rather deep water off Aberdeen ; 

 sometimes brought up by the lines, and not unfrequently cast 

 on the beach. Found also at Gamrie, in BanfFshire, by Miss 

 Macgillivray, and at Cruden by Mr. Murray. 



Yenus undata. Penn. Brit. Zool. 95. PL 55. f. 51. Lucina un- 

 data. Turt. Brit Biv. 115. Venus undata. Mont. Test Brit 117. 

 Venus undata. Flem. Brit Anim. 448. Lucina uiidata. Lamk. 

 Syst v. 543 ; Ed. 2. vi. 229. 



GENUS 5. VENUS. 



Shell equivalve, inequilateral, moderately convex, sub- 

 orbicular, sub triangular, or cordato-ovate, concentrically 

 striate or laminate. Umbones prominent, approximated, 

 curved forwards. Frontal slope concave, with a dis- 

 tinct cordate depression. Hinge strong ; the right valve 

 with three divergent teeth ; the two anterior smaller, 

 separated by a narrow depression, the posterior elon- 

 gated separated by a wide triangular depression, and 

 succeeded by an elongated marginal groove ; the left 

 valve with three divergent teeth, of which the medial is 

 largest, three depressions, and submarginal grooves. 

 Ligament external, considerably sunk and inconspicu- 

 ous. Muscular impressions moderate, roundish, sub- 

 marginal; pallial impression invaded by an oblong 

 sinus. 



