BIVALVES. 



403 



united along the dorsal margin, but they are never connected on the ventral side. 

 Bivalves clothe their shells with a more or less distinct periostracum, which is some- 

 times thin, smooth, and shining, and often of a yellowish or olivaceous tint, or it 

 may be thick, pilose, velvety, or rugged. It has been already noticed that the 

 valves are nearly always connected dorsally by a ligament. This is not, however, 

 the means by which they are held together, or closed with such force ; this closure 

 being effected by one or two muscles (adductors), firmly attached to the inner 

 surface of the valves, and endowed with such power of contraction, that it is an 

 impossibility to force them apart with- 

 out injury. The places of attachment 

 of these muscles (m, m') are generally 

 visible, as well as other minor scars 

 caused by the pedal retractor muscles. 

 The point of attachment of the edge 

 of the mantle, known as the pallial 

 impression, is often quite distinct ; it is 

 parallel with the lower margin of the 

 valves, and, in some groups, is more or 

 less deeply sinuated (ri) below the 

 posterior adductor impression. All LEFT VALV E OF Meretrix. 



pelecypods are aquatic, the majority a, Anterior; b, Posterior end; c, Umbo; d, Ventral 



being marine. They are less numerous ma l gi ^. ; < Anterior adductor scar ; m '> Posterior > 



J n, Pallial sinus. 



than gastropods in species, but in in- 

 dividuals are at present, as in past ages, relatively far more so. They are found at 

 all depths, from low- water mark ; many having been dredged in more than two 

 thousand fathoms. Bivalves, however, are most abundant in shallow water. They 

 live buried in the sand or mud, or attached to rocks and other substances, either by 

 the shells themselves, or by means of a byssus, consisting of horny fibres secreted by 

 a gland near the extremity of the foot. Others bore into rocks, wood, and other 

 substances, and a few take up their abode in the tests of certain Tunicata, and in 

 sponges, in the grooves of sea-urchins ; and one species (Entovalva) lives parasiti- 

 cally inside a sea-cucumber. Dr. Pelseneer divides the class into five orders, based 

 mainly upon the structure and morphology of the gills, but at the same time upon 

 the general conformation of the animal. To explain in detail the differentiating 

 anatomical characteristics of these orders would be beyond the scope of the present 

 work, and consequently only salient features can be mentioned. 



Order PKOTOBRANCHIATA. 



In these bivalves the gills have simple unreflexed filaments, disposed in two 

 rows in opposite directions; the foot being expanded, with crenulated margins, 

 and with scarcely any byssal gland. The families Nuculidce and Solenomyidce 

 constitute this order. The molluscs belonging to the former are all marine ; and 

 have the mantle free all round, or forming two small posterior siphons. The gills 

 are small, but the labial palpi very large. In the typical Nucula the shell is 

 small, more or less triangular, generally covered with a greenish olive periostracum 



