Conchology. 71 



when this is determined, every other part will fall progres- 

 sively in their relative order under our observation. We name 

 that part of the margin or limb which is situated in a direct 

 line opposite the beak, the base of the shell. 



Base of the bivalve shells, exemplified in Venus verrucosa, 

 fig. 5, V. 



Summit, a word applied in a general manner to the top or 

 most elevated part of the two protuberances observable in the 

 greater number of bivalves, fig. 5, s. 



Beak, the pointed termination, apex, or tip of the protu- 

 berances last mentioned, and which, in many shells, turn 

 spirally downwards, or obliquely, so that the beak itself is sel- 

 dom the most elevated part of the shell. 



Sides, the lateral parts of the valves distinguished by the 

 epithets of right and left sides; in common language, the two 

 valves of a shell are called the sides, but it is not understood 

 as a term in conchology in this view, fig. 5, x, z. 



Margin or limb, the whole circumference or outline of the 

 shell, when laid flat down on one valve. 



Disk, the convex centre of each valve or exterior surface, 

 fig. 6, d. 



Anterior slope, that part of the shell in which the ligament 

 is situated ; in the front view of the anterior slope, the beak 

 falls back or behind, fig. 6, e. 



Posterior slope, that immediately opposed to the former^ 

 and in which the beaks of the shell turn forward, fig. 6, c. 



Lunule, the lunulated depressions below the beaks, either 

 on the anterior or posterior slope, and sometimes on both ; 

 they may be distinguished under the appellation of anterior 

 or posterior lunules, according to the slope in which they are 

 situated, fig. 7, a, b. 



Cartilage of the lungs, called also the ligament of the 

 hinge, the substance of a flexible, fibrous and somewhat 

 horny nature, by means of which the two valves are united 

 near the beak, fig. 8, c. 



Inside of the valve, exhibits the concave surface, fig. 7, i. 



Ears, the lateral processes near the beaks, as in the scallop 

 tribe : those occur either on one side, or on both, fig. 9, e, e. 



Length and breadth of the shell. The length is measured 

 from the cartilage or beak to the margin below; the breadth 

 is of course taken in the opposite direction. The breadth of 

 many bivalve shells exceed their length : some remarkable 

 instances of which occur in the solen tribe, fig. 8, a, a, length; 

 b, b, breadth. 



