252 MALACOZOA. TftOPIOPODA. LAMELLIBRANCHIATA* 



is seen passing out of it, and minute dark particles frequently 

 escape. In this manner the animal advances with considerable 

 speed, by jerks. At other times it ascends to the surface, 

 where it proceeds in the same manner, with the shell reversed, 

 the umbones being beneath. The valves being partially opened, 

 the foot is gradually elongated, until it assumes a linear, sub- 

 cylindrical form ; it then suddenly contracts and the shell is 

 jerked forward. Usually the foot extends to a length and a- 

 half of the shell, but often to twice its length. The syphon 

 changes little, being generally cylindrical, sometimes however 

 nearly elliptical, but always abrupt. It never extends beyond 

 a sixth of the length of the shell. The animal can advance 

 equally with the shell inclined to either side. It can creep in 

 any direction, on a level or inclined surface, ascend or descend 

 a perpendicular plane. The colour of the protruded parts is 

 greyish-white. On opening an individual in which young ones 

 were seen, I found six lodged in the umbonal region. They 

 were very large, a third of a twelfth long, much compressed, 

 elliptical, with the umbones nearly central, and scarcely ele- 

 vated above the general curve, the colour white, the surface 

 beautifully glossed, and almost perfectly smooth, the striae being 

 scarcely apparent. On being removed and placed in the water, 

 some of them soon began to move in the same manner as the 

 adults, but with the shell inclined on one side. 



Individuals taken from the Canal near Aberdeen, of a more 

 ovato- triangular form, and less bulging, presented no differences 

 in the foot and siphon, or in the mode of progression. 



Pisidium Jenynsii. Gray, Turt. Man. 2d Ed. 285. Pisidium 

 pulchellum. Var. Jenyns. 



3. Pisidium pulchellum. Beautiful Pisidium. 



Shell obliquely ovate, rather ventricose, very thin, glossy, 

 finely and deeply concentrically striate, with a few more marked 

 growth-lines, and with faint radiating striulse ; the umbones 

 tumid, obtuse, considerably nearer the anterior end ; frontal 

 end forming about the third of a circle ; dorsal slope descend- 

 ing, little convex ; the colour pale yellowish-grey, or greyish- 

 white. Length a twelfth and three-fourths, height nearly a 

 twelfth and a-half, breadth a twelfth and a-fourth. 



The animal reddish- white. 



Adult individuals vary little, unless in colour, some being of 

 a darker or olivaceous tint, others tinged with grey, many 

 crusted with reddish, brownish, or blackish matter. Young 



