38 INTRODUCTION. 



Regularity. 



A great number of Bivalves are extremely regular in their 

 form. These are generally locomotive, and consequently free 

 from those obstructions in growth occurring to stationary shells, 

 which being confined in a particular position, or to a parti- 

 cular spot, modify their shape according to the substance with 

 which they come in contact, and thus become irregular. This 

 is generally the case with shells which are attached to sub- 

 marine 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 line drawn from the umbones to the ventral margin would 

 divide the shell 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, as in Cardium cardissa (fig. 122 

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

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

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



