200 CEETACEOUS PELECYPODA 



XVI. Family— CYBENIJD^, 



The animals possess a thick fleshy mantle, especially along the edges, which are 

 entire ; the margins are open almost all round, except for a yery short distance 

 below the siphons ; these are two, distinctly developed, usually slightly united at 

 the base and merely with the orifices separated and fringed; sometimes, as in 

 SpJKBrium, the siphons are prolonged and distinctly separated towards their termi- 

 nations ; in a few species of Indian CotUguIcb the siphons are also somewhat pro- 

 duced, but only at the orifices separated, exactly as in Cyprina islandica, the 

 retractile muscles are very strong and of a semicircular shape. The foot is generally 

 moderately produced, laterally compressed, broad and attenuated towards its end, 

 below near the base provided with a byssal groove ; the palpi are two on either 

 side, rather narrow and lingui-form, finely striated ; the gills are in pairs, one on 

 each side, sub-equal, coarsely striated and large, occupying from i to -P- of the total 

 length of the body. 



The shells are sub-cordate, usually moderately inflated, strong and solid in the 

 brackish- water-, thinner in the fresh-water- species, always covered with an oliva- 

 ceous, brittle epidermis, the shell-surface itself being concentrically striated or sul- 

 cated ; hinge with two or three cardinal teeth in each valve and one lateral tooth on 

 either side, single in the left, double in the right valve ; ligament external ; pallial 

 impression very often with a small but not quite distinct posterior sinus, some- 

 times simple. 



The species referred to the present family may be looked upon as the brackish 

 and fresh- water representatives of the entire order VFNJEBACJEA, Their animals 

 very closely resemble those of Cyprina and other gzossid^, as exhibited by the brief 

 account given by Eischer of the anatomy of a Oyrena (Journ. de Conch., 1863, 

 p. 5). The anatomy of our Cyrena Bengalensis agrees with Eischer's account in 

 general, but I hope to give a fuller account of it at some future date. I have also 

 examined several species of Indian CorhiculcB, and as far as the form of the foot 

 and of the siphons is concerned, they are almost identical with Cyprina, On 

 the other hand, the shortness of the siphons, the almost entirely separated mantle- 

 margins, and the great size of the gills leaves no doubt that we have in the 

 CybenibjM a group of animals which form the passage from the Olossid^ to the 

 Casdiid^, 



All the recent species are characterized by a covering of a peculiar brittle 

 epidermis. In the brackish forms, such as Cyrena and Batissa, the beaks are 

 commonly found much eroded ; this is chiefly due to the influence of the ebb and 

 tide, the specimens remaining during low water very often for eight and nine hours 

 dry, or buried in soft mud. Such is also the case with old Corbiculce when they 

 live in tanks or streams where the water often dries out, but in young shells the 

 beaks are generally found perfect, because the shells mostly live between water 

 plants, and permanently in water. 



Temple Prime, who favoured this family with his special study, says (Monog. 

 Am. CoBBicuLAB^, Smith. Misc. Coll., No. 145, 1865, pp. 3, 12, &c.,) that the 



