THE BIVALVES 3373 



ligament, or resilium, and often interlocked at the same place by projections on 

 the edges of the valves, termed hinge teeth. In the majority of species the valves 

 shut closely together all round the edges, but in many they gape at one or both ends, 

 or at the ventral side. In certain species of Pinna the two valves are actually 

 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 (n) below the 

 posterior adductor impression. All LEFT VAI.VE OF Meretrix. 



pelecypods are aquatic, the majority Anterior; *. Posterior end ; c. Umbo; d. Ventran 

 * margin ; m. Antenor adductor scar ; m'. Posterior ; 



being marine. They are less numerous . p a iiiai 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 parasit- 

 ically 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 PROTOBRANCHIATA 



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

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



