582 Anatomy of the Lamellibranchiata. 



particular form of a valve is not dependant on the greater or 

 less protrusion of the lamina at different points, or their great- 

 er or less distance from each other, or on their being depo- 

 sited at a greater or less angle with those preceding, but is the 

 same at all ages, depending upon the specific configuration 

 of the animal. We do, however, see slight differences of form 

 in the young and old animal. In the fry of the Cyclades for 

 instance, the valves are much flatter than in the mature state, 

 and the posterior extremity of young shells generally is little 

 developed : the openings of those shells which, in the mature 

 state, are naturally gaping, are often less apparent in the 

 young, from the imperfect developement of the foot, siphons, 

 &c, which protrude by them. In the Tridacna on the con- 

 trary, the opening of the shell becomes closed in the mature 

 animal.* 



The valves differ infinitely in form in the different species. 

 Sometimes the valve is longest in the vertical direction, as 

 in Lima, Malleus, &c. ; sometimes of the greatest diameter 

 in the opposite horizontal direction, as in Solen. In the ge- 

 nus Solemya the greatest side of the valve is anterior to the 

 beaks, or commencing points of growth; whilst in others, 

 as in many Solenides, the valves are totally post-apical. — 

 When both sides of the same valve are equal, it is termed 

 equilateral ; when they are not perfectly alike it is inequila- 

 teral^ The beaks look towards the shortest side of the 

 valves ; thus they look forwards in Venus and backwards in 

 Donax. The figure and size of the foot influence most ma- 

 terially the form of the anterior part of the shell ; the con- 

 formation of the siphons that of the posterior part. Thus in 

 Donax the anterior part of the valves is large, to protect and 

 encase the voluminous foot. The orbicular species of Venus 

 have a broad securiform foot; the oval species have it oval or 

 lanceolate ; the cylindrically valved animals elongated or 

 club-shaped. 



We see the same ridges, strife, laminae, spines, &c, in bi- 

 valves as in other shells, marking the greater activity at cer- 

 tain times of the mantle. % An important point in Conchology 

 is the beaks being remote or proximate. The former circum- 



* Blainville. 



f The reversion of the shell, so common in univalves, occasionally occurs 

 in bivalves. Gray, ' Zool. Journ.' vol. i. 



% On examining- the surface of some shells with a lens, we notice fine 

 punctuations, as m the Pinna, Avicula, and certain Brachiopoda ; also fine 

 striee, as in many Tellina, where they are often oblique, and sometimes only 

 on one valve. In the Tel.fabula, Fleming says these are on the left valve; 

 in the Sussex specimens they are on the right. 



