MALACOZOA. TROPIOPODA. LAMELLIBRANCIIIATA. 209 



finely striate ; the epidermis pale greyish-yellow. Fla- 

 vescens, yellowish. 



Genus 2. Pisidium. — Shell suboval, subtrigonal, veiy 

 thin ; umbones tumid, nearer one end ; siphons united. 

 Fisum, a pea. Scopoli, Pfeiffer. 



1. Pisidium Joannis. — Shell broadly ovate, mode- 

 rately convex, thin, glossy, distinctly concentrically stri- 

 ate, with more marked growth-hnes ; the umbones 

 tumid, obtuse, smooth at the apex, somewhat nearer the 

 anterior end ; the dorsal slope a httle convex, both ends 

 rounded ; the colour gi-eyish-yellow. Named after Mr. 

 John Macgilhvray. 



2. Pisidium Jenynsii. — Shell obliquely ovate, mode- 

 rately convex, very thin, glossy, finely but distinctly and 

 regularly concentrically striate, with a few more marked 

 growth-lines ; the umbones tumid, obtuse, considerably 

 nearer the anterior end ; the dorsal slope about a third 

 longer than the frontal, and convex ; the colour yellow- 

 ish-white. Named by Mr. Gray after the Rev. Leonard 

 Jenyns, of Cambridge. 



3. Pisidium pulchellum. — Shell obliquely ovate, rather 

 ventricose, very thin, glossy, finely and deeply concen- 

 trically striate, with a few more marked grov.th-hnes, 

 and with faint radiating striulse ; the umbones tumid, 

 obtuse, considerably nearer the anterior end ; the dorsal 

 slope descending, little convex ; the colour pale yellowish- 

 grey, or yellowish- white. PulchcUus, small and beau- 

 tiful. 



4. Pisidium nitidma. — Shell orbicular-ovate, com- 

 pressed, thin, glossy, finely and regularly concentrically 

 striate, with a few more marked growth-lines ; the um- 

 bones obtuse, subcentral, marked with several stronger 

 grooves ; the colour greyish-white. Nitidus, shining. 



5. Pisidium pusillum. — Shell orbicular-ovate, rather 

 compressed, thin, glossy, finely concentrically striate ; 

 the umbones obtuse, rather prominent, subcentral ; the 

 dorsal slope descending and convex ; the colour greyish- 

 white. Pusillus, dwarfish. 



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