462 TERMINOLOGY OF 



the anterior slope is equal to the posterior and of a like form, 

 the shell is symmetrical. To be strictly so, a bivalve should 

 in reality, have four similar areas; but this perfect symme- 

 try is not to be found excepting in some genera of Brachio- 

 pods. There is, however, sufficient equality in the areas of 

 Pectunculus, &c., to allow them to be described as symme- 

 trical. 



When a line drawn from the umbonal region to the base 

 divides the valve into two equal, or nearly equal, halves, the 

 shell is equilateral ; and when one side is decidedly longer 

 than the other, the shell is inequilateral. 



If, when closed, the margins of the valves meet in contact, 

 the shell is close; but if the margins do not meet in any point 

 of the circumference, the shell is gaping. The gape is com- 

 monly on the posterior slope ; and it is not uncommon to 

 find a gape in both slopes or ends, as in Solen. 



A shell is auricled or eared when an appendage or process 

 is extended from one or both sides of the apices and sepa- 

 rated from them by a line or suture ; e.g., Pecten ; and when 

 this lateral process is more extended and appears to be con- 

 tinuous with the beaks, the shell is said to be lobed or to 

 have lateral lobes. When the beaks are prolonged forward, 

 beyond the outline of the valves, the shell is rostrated. The 

 Oyster and other cemented shells frequently become ros- 

 trated. 



When any part of the outline is abruptly interrupted in 

 the circle that it would, if continued, have described, the 

 shell is truncate. You have examples in Donax, in Cardium, 

 and in Mya, &c. 



When a shell is elongated and swollen equally on both 

 sides, it becomes cylindrical, as in Lithodomus ; but if the 

 valves are, on the contrary, flattened and almost plane, the 

 shell is compressed. If round in the circumference a com- 

 pressed shell is also orbicular; and if the valves are tumid, 

 the shell is globular, or globose. The terms heartshaped, lenti- 

 cular, navicular, rhomboidal, and tongueshaped, explain them- 

 selves. A linear shell is an elongated shell with the supe- 

 rior and inferior margins parallel to each other and straight, 

 for if the line bends, the shell is arched or arcuated. The 

 genus Solen affords examples of both structures. 



Each valve of a bivalve shell has two surfaces an external 

 and an internal one. 



The external surface is covered with the Periostracum or 

 Epidermis varying much in thickness in different genera, and 

 in some so thin and inseparable as to be detected with diffi- 

 culty. It is then said to be obsolete, or wanting. It may be 



