MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 249 



First found by me in September, 1842, in a ditch near the 

 Links of Old Aberdeen, where it is extremely abundant among 

 the roots of grasses and other plants. Individuals kept in 

 water for some days presented the same habits as those de- 

 scribed in Pisidium Jenynsii, with the exception of not extend- 

 ing the foot nearly so far. 



With reference to specimens of this shell, Mr. Gray, to 

 whom I sent them, remarks : "The Pisidium is very like one 

 I have lately received from Yorkshire, without a name, and 

 appears distinct from any others I have." Supposing it to be 

 new, I have named it after my son, a young naturalist of some 

 promise. But, although I am persuaded that it equally merits 

 specific distinction with Pisidia Jenynsii, and pusilla, I think 

 that they all differ as little from each other as the numerous 

 varieties of Linna3i pereger, palustris, and truncatulus, or of 

 Mytilus edulis, or Venus Gallina. 



In still water, full of decayed vegetable matter, in a small 

 neglected pond, and especially in a ditch near it, in the garden 

 of the Professor of Medicine of King's College, where it was 

 first observed by Mr. Leslie, in June, 1 842, it presents a some- 

 what different appearance, so that one might at first sight 

 consider it a distinct species. Although more or less crusted, 

 it is pale greyish-yellow when cleaned. Most of the old indi- 

 viduals resemble those above described ; but are more convex, 

 and the largest, which attain a length of two- twelfths and a 

 quarter, are ventricose, with the nates considerably elevated, 

 often capped, and the inferior margin meeting at a much wider 

 angle. Many of the individuals have a slight tinge of grey, 

 and the animal is often greyish-white. 



Respecting specimens from this locality, Mr. Jenyns has 

 favoured me with the following remarks : " This I believe to 

 be the P. pulchelhim, var. a of my monograph, which I have 

 since considered distinct from the above (P. pulchellum, var. 

 /3), and which Gray (p. 285) proposes should be named P. 

 Jenymii ; but they are much larger than any specimens in my 

 possession before, with the umbones more prominent, and the 

 valves less deeply striated ; but these differences may be owing 

 to water and locality. If it is not what I suppose it to be, it 

 is new." 



2. Pisidium Jenynsii. Jenyns 's Pisidium. 



Shell obliquely ovate, rather tumid, very thin, glossy, finely 

 but distinctly and regularly concentrically striate, with a few 

 more marked growth-lines, and with very faint radiating 



