250 MALACOXOA. TROPIOi'QDA. LAMELLIBRANCHIATA. 



striulae ; the umbones tumid, obtuse, considerably nearer the 

 anterior end ; frontal end forming about a third of the seg- 

 ment of a circle, dorsal slope about a third longer than frontal, 

 and convex; the colour yellowish-white. Length a twelfth 

 and three-fourths, height a twelfth and an eighth, breadth 

 three-fourths of a twelfth. 



The animal is yellowish-white, or reddish- white, semitrans- 

 parent. 



It resides among the roots of aquatic plants, especially 

 Junceae, Cyperaceae, Gramineae, Spargania, Epilobia, Veronica 

 Beccabunga, Callitriche verna and Sphagna ; generally among 

 the fibres, clear of the mud, but sometimes immersed in the 

 latter. As it inhabits lakes, pools, and ditches, clear, running, 

 stagnant, or muddy water, marshy places, and peat bogs, it 

 varies in size, form, and colour. Some individuals are less in- 

 equilateral than others ; some ventricose, others compressed. 

 Frequently the umbones are somewhat capped, or present a 

 distinct separation, as it were, from the rest of the shell, by a 

 sudden depression. Among specimens collected in a single 

 spot, a small pond, for example, many such varieties are to be 

 seen. They depend partly upon age, and partly upon local 

 circumstances. 



In the clear running water of a ditch, filled with plants, and 

 especially Callitriche verna, near Old Aberdeen, for example, 

 it presents the characters given above ; the largest individuals 

 being obliquely ovate, with the posterior end twice the length 

 of the anterior, the concentric striae regular and delicate, the 

 umbones moderately prominent, but always very obtuse, the 

 shell rather compressed, although tumid at the umbones, and 

 the valves meeting below so as to form an acute angle, of about 

 forty degrees. Younger individuals are proportionally less 

 elongated, more compressed, with less tumid umbones ; very 

 young ones are ovato-orbicular, much compressed, and with 

 the umbones scarcely prominent. The shell is always yellow- 

 ish-white, when cleaned, and when the animal is in it, it is 

 pale hyaline-yellow ; but it is often crusted with brown or red- 

 dish matter. When dead it is whitish and opaque. The 

 greatest size in such places is two-twelfths in length, a twelfth 

 and a-half in height, and nearly a twelfth in breadth or thick- 

 ness. 



In clear, but stagnant water, in a ditch, in gravelly ground 

 with peaty soil, between Aberdeen and the Spital, along with 

 Plariorbis Vortex, I found in July, 1842, a most beautiful 

 variety; oval, ventricose, with prominent obtuse nates, the 



