280 DIMYARIA.—DONACIDA. 
tants of sandy shores in most seas, and the majority 
of them are found in the vicinity of low-water mark. 
Many of them are of a large size. We have a Bri- 
tish species, Mactra helvacea, which attains the size 
of three inches by two and a half in diameter. 
This, however, is one of the rarest of our native 
shells, as JZ. stultorum is one of the most abundant. 
The latter is found on every sandy beach; it is 
about half the size just mentioned, somewhat tri- 
angular in outline, and varying much in colour, 
from plain drab to fawn-colour, marked with pale 
zones and white divergent rays. 
FamiILty DoNACID. 
The shell is more or less triangular and com- 
pressed, broader than long, equal-valved, but un- 
equal-sided, the hinder side being the shorter. 
The texture is strong and compact, the hinge 
variable, but always provided with conspicuous 
primary teeth. 
The animal is generally richly coloured, with 
the mantle usually fringed; the siphonal tubes 
are separate and much: developed, having their 
orifices adorned with deeply-cut tentacles. The 
foot is large, compressed, sharp-edged, and angular ; 
the gills very unequal. 
The species of this family, which are rather 
numerous, occur in all parts of the world, but most 
abundantly in the Southern Hemisphere. They 
live buried in sand or sandy mud, with the short 
side of the shell uppermost, the siphons being pro- 
truded, and range from low-water mark to a depth 
of ten fathoms. 
