MALACOZOA. TROPIOPODA. LAMEL1IBRANCHIATA. 253 



individuals are more elliptical, with the anterior end less 

 shortened. 



Found in ahundance in the mud and sand of a mill-pond 

 near the New Bridge of Don. First seen by Mr, John Mae- 

 gillivray there in the Autumn of 1841. 



Pisidium pulchellum,Var. /3. Jenyns, Monogr. 18. PI. 21. f. 1. 

 Cyclas pusilla. Turt. Brit. Biv. 251. PI. 11. f. 16, 17. Cyclas fon- 

 tinalis. Brown, Edinb. Journ. of Nat. and Geogr. Sc. I. 11. PI. 2. 

 f. 5, 7. 



4. Pisidium nitidum. Shining Pisidium* 



Shell orbicular-ovate, compressed, thin, glossy, finely but 

 distinctly and regularly concentrically striate, with a few more 

 marked growth-lines ; the umbones obtuse, subcentral marked 

 with several stronger concentric grooves ; the frontal end semi- 

 circular, the dorsal slope convex ; the colour greyish-white. 

 Length a twelfth of an inch ; height a fifth less. 



The animal greyish-white. 



First found by me in July, 1842, in a mill-pond, near the 

 New Bridge of Don, where it occurs rarely, along with Pisi- 

 dium pusillum, and P. pulchellum. 



The most obvious distinctive character consists of the strong 

 concentric ridges on the umbonal region. 



Pisidium nitidum. Jenyns, Monogr. 16. PI. 20. f. 7, 8. Pisidium 

 nitidum. Gray's Turton, 283. PL 12. f. 150. 



5. Pisidium pusillum. Dwarf Pisidium. 



Shell orbicular-ovate, rather compressed, thin, glossy, finely 

 concentrically striated ; the umbones obtuse, rather prominent, 

 subcentral ; the frontal end forming the fourth of a circle, the 

 dorsal slope descending and convex ; the colour greyish-white. 

 Length a twelfth and a-fourth, height nearly a twelfth. 



The animal greyish-white. 



First found by me, in the Summer of 1841, in a mill-pond, 

 near the New Bridge of Don ; in April, 1842, gathered there 

 in abundance by Mr. Leslie and me. 



This species differs from the last chiefly in being less com- 

 pressed, and in wanting the prominent ridges on the umbonal 

 region. 



Cyclas pusilla. Turt. Brit. Biv. 251. PI. 11. f. 16, 17. Cyclas 

 fontinalis. Drap. Moll. J30. PI. 10. f. 8, 11. Pisidium pusillum. 

 Jenyns, Monogr. 14. PI. 20. f. 4, 6. Pisidium pusillum. Gray's 

 Turton. 284. PI. 12. f. 151. 



