3380 THE MOLLUSKS OR SHELLFISH 



and the single adductor muscle in the adult is eccentric. They are generally her- 

 maphrodite, but sometimes the sexes are separate. More than a hundred species 

 from all parts of the world and all depths have been described. Ten occur on the 

 British coasts, and fossil species are numerous in all formations, from the Carbonif- 

 erous. The file shells (Limidce) somewhat resemble the scallops, but are nearly 

 always white, and the edges of the mantle, which have no eyes, are furnished 

 with long tentacular filaments. Some swim freely by flapping their valves, 

 others attach themselves by a byssus, or, as in the figure on p. 3379, 

 construct a nest of broken shells, stones, and other substances held together 

 by a network of byssal threads. The two largest species occur off the coasts of 

 Norway and Japan. The recent species of the chief genus (Lima} are not numer- 

 ous, and some occur at great depths. The fossil forms, on the contrary, are more 

 abundant, from the Carboniferous age, and some of the species from the L,ias 

 (Plagiostoma) are of very large size. The members of the allied family Spondylidce 

 are known popularly as thorny oysters, on account of the spiny characteristic of 

 their surface ornamentation. In general shape they are rather like the pectens, and 

 similarly brilliantly colored; but they have much more solid shells, and the hinge 

 consists of powerful interlocking teeth, while the animal has 'no byssus, a more 

 rudimentary foot, and lives with a few exceptions, attached by one of the valves to 

 rocks and stones. The ligament of Spondylus is internal. The single adductor 

 muscle is a little eccentric, and the mantle margin has a row of eyes. 



Order EULAMELUBRANCHIATA 



In this order the gills have vascular, interfilamentary, and interfoliary junc- 

 tions; and the mantle is always united at one or more points, and there are generally 

 two adductor muscles. The order is the largest of all, and comprises nearly sixty 

 different families, of which only the most important or remarkable can be men- 

 tioned. In the first (Submytilacea) of several suborders into which the order is 

 divided, mention may be made of a curious little species of the family Carditidce, 

 namely, Thecalia concamerata, which is a native of South Africa, and remarkable 

 for a cup-like process formed by the female within the ventral margin of the valves, 

 serving as a nursing pouch for the young. Milneria minima, a Californian species, 

 forms a similar marsupium. In the family CyprinuUe, Isocardia cor is one of the 

 finest of the British bivalves; and is a large strong globose shell, with the umbones 

 prominently curved anteriorly. The ligament is external, and the hinge teeth are 

 strong and of peculiar form. The animal has short siphons, large gills, and a small 

 foot for burrowing in the sand. In the Lutinidce the shells are mostly white, round, 

 globose, or compressed, and peculiar on account of the great length of the anterior 

 muscular scar, which falls within the uninterrupted pallial line. Sometimes the 

 animals have only a single branchial lamella, and the foot is generally slender and 

 without byssus. The families Leptonidce, Galeommidce, and Chlamydoconchidce also 

 belong to this order. Lepton often lives commensally with Crustacea, Galeomma 

 has the mantle reflected over a considerable part of the valves, and in Chlamydo- 

 concha the shell is wholly covered by the mantle, a unique feature among the 



