Pvn H,i I'UitlS

TELLINA. 373
and Sars have made similar observations with regard to
the Crustacea and other Invertebrata which are common
to northern and southern latitudes. An explana-
tion (although perhaps an imperfect and unsatisfactory
one)
of this law has been offered in the l
to my first volume, p. xxxii.
Introduction *
The synonyms of this pretty species are few and ob-
solete. I am aware of two only, viz. T. orbiculata, Re-
nier, and T. serratula, Chiereghini.
2. T. crassa *, Gmelin.
Venus crassa, Linn. Syst. Nat. (ed. Gmel.) p. 3288. T. vrassa, F. & H. i.
p. 288, pi. xx. f. 1,2.
Body yellowish-white : mantle thick ; edges closely fimbri-
ated : tubes funnel-shaped and long ;
and fringed with 6 short tentacular cirri ; the excretal or
orifice of each narrow,
upper tube in an individual examined by me was three or four
times the length of the other tube ; mouth of the alimentary
tube much contracted and like a snout in shape :
gills nearly
circular, of a very thin texture ; lower pair twice the size of
the upper ; both are coarsely but not distinctly pectinated :
iMilps narrow, slender, pointed, of an elongated triangular form,
quite smooth externally, but conspicuously striated on the inner
side : liver dark brownish-green : foot very large.
Shell obtusely triangular rather than oval, compressed,
thick, opaque, moderately glossy ; the left valve is somewhat
larger and deeper than the other: sculpture, numerous strong,
threadlike, concentric ribs, which are more crowded and become
the narrow interstices
laminar at the sides also in the young ;
of these ribs are impressed with finer parallel striae, and crossed
by deeper, more regular, and close-set longitudinal striae, none
of which, however, are visible except by the aid of a magnifier
: colour the same as in the last : species epidermis fibrous,
yellowish-brown, slight, and effaced in nearly every part by
continual friction : margins curved in front, and obtusely an-
gular with rounded points at each end ; posterior side decidedly
valve : beaks small, incurved,
fiexuous, especially in the right
* Solid.