MALACOZOA. TROPIOPODA. LAMELLIBBANCHIATA. 'ZJO 



individuals are more elliptical, with the anterior end less 

 shortened. 



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

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

 gillivray there in the Autimin of 1841* 



Pisidmm pulchellum,Yar, (3. Jenyns, Moiiogr. 18. PI. 21. f. 1. — 

 Cyclaspiisilla. Tiu-t. Brit. Bir. 251. PI. 11. f. 16, 17.— Cyclas fon- 

 tinalis. Brown, Edinb. Journ. of Nat. and Geogr. So. I. 11. PL 2. 

 f. 5, 7. 



4. Plsid'ium 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. Jenvns, Monogi*. 16. PI. 20. f. 7, 8. — Pisidium' 

 nitidum. Gray's Turtou, 283. PI. 12. f. 150. 



5 Pisidiiun 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 fovmd by me, in the Summer of 1 841 , in a mill-pond, 

 near the New Bridge of Don ; in April, 1812, 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. PL 11. f. 16, 17.— Cyclas 

 fontinalis. Drap. MolL 130. PL 10. f. 8, 1 1.— Pisidium pusillum. 

 Jenvns, Monogr. 14. PL 20. f. 4, 6. — Pisidium pusillum. Grav's 

 Turton. 284. PL 12. f. 151. 



Y 



