BIVALVE SHELLS. 461 



Bivalve shells are said to be free, when the animals have 

 the power of locomotion and can displace themselves ; and 

 they are fixed, when, attached to a foreign base, they cannot 

 remove from their site. The fixed bivalves are byssiferous 

 when attached through the medium of a byssus ; peduncu- 

 lated, when moored by a membranous or fleshy pedicle ; and 

 cemented when adherent by the lower valve being directly 

 glued to the foreign base. 



Bivalve shells are equivalve, when the valves are alike in 

 size and form ; and inequivalve, when one valve is less than 

 the other. This inequality is sometimes so great that the 

 lesser valve looks like an operculum to the larger, and it is 

 then described as being operculiform. This is only found 

 in shells adherent from cementation, as in some species of 

 Ostrea, in the Hippurites, and the Spherulites. The com- 

 mon oyster and the Spondyli are the best examples of ine- 

 quivalve shells. A few only of free shells are inequivalve, 

 viz., some Anatinae, the genus Corbula, and a few Tellinae ; 

 and in these cases the lesser valve is not only less concave 

 but smaller in its circumference than the other. 



A regular shell is that which is the same in figure in all 

 the individuals of its species. An irregular shell is one that 

 is modified and altered more or less in its outline by external 

 influences, so that the individuals of the same species are 

 often unlike in contour and sculpture. The Veneridae, Tel- 

 linae, &c. are regular shells and equivalve ; the Ostreae are 

 irregular and inequivalve ; and the Placunae are irregular 

 but equivalve. 



The length of the bivalve is measured in the direction of a 

 line drawn from the apices to the base or lower margin. 

 (Fig. 84, oo). The shell is then said to be longitudinal 

 when it is longer than its transverse diameter, e.g., Mytilus, 

 Pinna ; and it is transverse when the latter diameter exceeds 

 the length, as is instanced in many shells, e.g. in Tellinae, and 

 is carried to excess in the Solenes. The depth of the shell 

 is measured by a straight line drawn from the centre of one 

 valve to the centre of the other. 



When a bivalve is placed on its base, with the ligament 

 or posterior slope towards the observer, the right and left 

 valve will correspond with its own sides. 



When the beaks look to each other and correspond, and 



views of the author whose works we are engaged in consulting." Zool. 

 Journ. i. 207. Instruments have been invented for measuring shells. Mr. 

 Gray uses a pair of callipers, and D'Orbigny an instrument which he calls a 

 helicometer. Ray Reports on Zoology, 1845, p. 116. They may be dis- 

 pensed with. 



