BIVALVES. 
4 i 3 
and large labial palpi and foot. The shells are nearly always compressed, subequi- 
valve, joined by an external ligament, and furnished with hinge-teeth. The scars 
of the adductors are far apart, and the pallial impression is generally very deeply 
sinuated. None of the bivalves are more beautifully coloured than the tellens, 
the prevailing tints being purple-red, crimson, and various shades of yellow. 
The surface-sculpture of the valves is often beautiful and delicate. Tellens live 
in sand or mud at slight depths in every sea, and the species may be counted in 
hundreds. Nine species of Tellina are British. The Scrobiculariidce form a 
smaller and less showy family than the preceding, the shells being united by an 
internal ligament, and sometimes by an external one also. The soft-parts are 
like those of the Tellinidce. Some species of the genus Abra have been dredged 
at enormous depths, both in the Atlantic and Pacific, but the majority of the 
family have been obtained in comparatively shallow water. The British Scrobi- 
cularia piperata buries itself in the mud of estuaries, and can extend its siphons 
five or six times the length of its shell. Although in some respects the wedge- 
shells ( Donacidce ) agree with the Tellinidce, they differ as regards the gills. In 
Donax the shells are of a triangular or wedge-shape, and have the inner margin 
of the valves crenulated. They are united by an external ligament, and furnished 
with cardinal and sometimes lateral teeth. The wedge-shells live buried in the 
sand in shallow water in warm regions all over the world, and three species occur 
on the British coasts. The genus Tphigenia, which has no lateral teeth, inhabits 
estuaries on the coast of Africa, Brazil, the West Indies, and Central America. 
In the mactras (Mactridce) the shells are often more or less triangular, and 
have an internal ligament, the siphons being united the entire length, and 
fringed at the ends. About one hundred and fifty species of Mactra are 
known. They occur on sandy shores in most parts of the world at shallow 
depths, six being British. This family comprises a large number of genera and 
subgenera, mainly distinguished by modifications of the hinges; Mulinia, 
Spisula, Standella, Rangia, Rceta, and Eastonia being the more important. The 
remaining familes, Mesodesmatidce and Cardiliiclce, are not of special importance, 
and may be passed without further reference. 
Suborder Veneracea. 
In the Veneridce, which form the first family of this group, the animal 
has rather short, more or less united siphons, with fringed openings. The 
mantle is open in front for the passage of a tongue-like foot, which is sometimes 
furnished with a byssus. The shells are solid, equivalve, and often beautifully 
coloured and sculptured, with the hinge-teeth large and divergent, and the 
ligament external. In the typical genus Venus the shells are more or less 
circular, globose, and often have the surface cancellated, and the inner edge of the 
valves crenulated. There are several British forms of Veneridce, but none are 
eaten to any great extent. On the contrary, the large Venus mercanaria —the 
clam of the Atlantic States of North America—is sold in large quantities in 
the fish-markets of New York and Philadelphia. In Dosinia the shells are 
more compressed, circular, and marked within with a deep, narrow, pallial 
