EXPLANATION OF THE PARTS OF SHELLS. 11 



is situated. In viewing the posterior slope in front, the beaks of 

 the shell retire from view. Plate T. fig. 4, i. 



interior slope, that part of the shell opposite the posterior 

 slope ; in viewing it in front, the beaks point to the observer. 

 Plate 1. fig. 4, A7. 



Disk, the convex centre of a valve, or most prominent part of 

 the valve, suppose it with its inside lying undermost. Plate I. 

 fig. 4, 0. 



Inside, the concave part of a valve. Plate IV. fig. 6, m. 



JSIuscular impression is the impression left on the inside of 

 the valves, by the adhering muscles of the animal. It diflfers in 

 most shells, according to the shape of the animal, as semi-ovate, 

 round, lunate, elongated, &c. As a specific distinction, it is often 

 of great use ; being, with a very few exceptions, alike in shells 

 of the same species. Some shells have only one cicatrix, as the 

 Edible Oyster and Mytilus ; others have two, and some few more ; 

 the Tellina for example. Plate I. fig. 6 and 8, e e e e. 



Lunule. The lunated depressions, situated in the anterior and 

 posterior slopes. In different species of Venus they are promi- 

 nent characteristic marks, often of much service in ascertaining a 

 species. Plate I. fig. 4, b b. 



Ligament perforation. The circular aperture, or perforation 

 through which the ligament passes ; by which the animal of the 

 Anomia attaches itself to stones and other marine extraneous 

 bodies, it is in general situated in the flat valve, though there are 

 a few exceptions to the contrary. Plate I. fig. 7, m. 



Hinge is the point at which bivalve shells are united ; it is 

 formed by the teeth of one valve inserting themselves between 

 those of the other, or by the teeth of one valve fitting into the 

 cavities or sockets of the opposite one. 



It is on the peculiar construction of the hinge that the generic 

 character of bivalve shells is principally founded, together with the 

 general contour of the shell. Plate II. fig. 1 and 2, q q q. 



Teeth of the hinge. Upon the number and relative situation 

 of the teeth principally depend the specific distinctions ; they are 

 of various forms, and very differently placed ; some are single 

 and large, others numerous and small, orbicular, spatuliform, la- 



