38 INTRODUCTION. 



Regularity. 

 K great number of Bivalves are extremely regular in their 

 form. Their animals are generally locomotivOj and the shells 

 consequently free from those obstructions in growth which 

 occur to stationary shells. The latter, being confined to a 

 particular position, or particular spot, modify their shape ac- 

 cording to the substance with which they come in contact, 

 and thus become irregular. This is generally the case with 

 shells which are attached to submarine substances, such as 

 Spondyli, Oysters, &c. ; and the degree of irregularity will 

 depend upon the extent of surface involved in the attachment. 

 In the case of fixed shells, the attached valve is usually termed 

 the under valve, and the other which moves freely upon the 

 hinge, is termed the upper valve. 



Form and Proportions. 



Bivalves are said to be equivalve when the two valves cor- 

 respond in extent, breadth, and thickness ; and of course 

 inequivalve in the contrary case. They are equilateral when 

 a hne drawn from the lunbones to the ventral margin would 

 divide the sheU into two nearly equal parts ; and of course 

 inequilateral in the opposite case, which occurs in the great 

 majority of instances. 



A Bivalve is said to be compressed, when the distance is 

 small from the most prominent part of one valve to that of the 

 other. It is cylindrical when lengthened, and more or less 

 rounded in its breadth, as in Lithodomus (fig. 161 in the 

 plates.) It is cordiform when the shape presents a resem- 

 blance to an imaginary heart, asinCardium cardissa (fig. 122 

 in the plates), and in the Isocardia (fig. 126 in the plates). It 

 is lingiiiform when it resembles a tongue in shape, as in 

 Vulsella (fig. 185 in the plates) ; rostrated when it protrudes 



