3 o6 LOWER FORMS OF MARINE LIFE 



European Pholas dactylics (which is also South African), the two valves remain 

 separate throughout life, and are supplemented by additional valves. Piddocks 

 form vertical tunnels in rocks varying in hardness from mud to limestone. The 

 common European gaper (Mya arenaria) typifies another group of boring bivalves, 

 which dig a foot or so deep into sandy or muddy sea-bottoms. The shell, which is 

 ovate and inequivalve, gapes at both ends, and the siphons are long and united. 

 Four other genera are included in the Myidce, but the common European rock- 

 borer (Saxicava rugosa) typifies a second family, whose members range down to 

 a depth of 550 fathoms and bore only in soft stone. In the European species the 

 shell does not exceed 1£ inches in length, and is abruptly truncated at the hinder end. 

 Between this family and the one previously mentioned come the razor-shells 

 (Solenidce), which bore vertically into sand, and have shells resembling a scabbard, 

 either curved or straight, open at the two ends, and united for a part of one side 

 by a horny ligament. Ensis siliqua is a well-known species common to Europe 

 and North America. The tellins (Tellinidai) are distinguished by having the shell 

 compressed, with the valves equal and the ligament external. In the European 

 Tellina balthica the shell is pointed at the hinder end, with the margin rounded in 

 front and the hinge-line curved, the colour being reddish or yellowish banded with 

 white. Two of the North Atlantic tellins, T.fabula and T. tenuis, are also found in 

 South African waters. In the Venus-shells {Veneridce) the hinge has usually 

 three long diverging teeth, the ligament is external, while the muscular scars in the 

 interior are oval and distinct, and the impression formed by the margin of the 

 lobes of mantle is curved. A well-known form is the clam (Venus mercenaria) 

 of the west shore of the North Atlantic, whose heart-shaped, straw-coloured shell 

 is spotted internally with violet. Clam-shell discs strung together were formerly 

 used by the Indians for currency and other purposes, under the name of wampum. 



The scallop-shells (Pecten) as a rule swim freely and strongly by flapping the 

 valves of their shells together. They have a more or less circular equilateral shell, 

 with prominent ears, and when the valves are not equal rest on the bottom with 

 the flatter one uppermost. Two of the common British species — P. maximus, the 

 scallop, and P. opercularis, the quin — are edible. The pilgrim-shell, P. jacobceus, 

 famous as having been worn in the hats of the pilgrims to the Holy Land, resembles 

 P. maximus generally, but has the ribs of the lower valve angulated instead of 

 rounded. The file-shells (Lima) form a closely related family, in which the valves 

 are equal and compressed and the shell is obliquely oval with the anterior side 

 straight and gaping. It is nearly always white. Some of the species build nests 

 of broken shells and other fragments in which they become enclosed and anchor 

 themselves by the byssus ; but other species are free. The commonest British 

 species (L. hians) is conspicuous for the bright orange colour of its mantle lobes. 



In the next family (Spondylidai) there is no byssus, but the foot has an ap- 

 pendage, the shell is usually spinose, and there are two cardinal teeth in each valve. 

 The red oyster of the Mediterranean (Spondylus gcederopus) is perhaps the best 

 known representative of the family. The oyster-family also belongs to this order ; 

 one of the most familiar species in Europe being the common oyster (Ostrea edulis), 

 which is confined to the eastern side of the North Atlantic and most numerous in 

 the North Sea, though there are about seventy other species distributed through- 



