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 I. fig. 4, 7. 
Anterior 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, k. 
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. 
Muscular impression is the impression left on the inside of 
the valves, by the adhering muscles of the animal. It differs in 
most shells, according to the shape of the animal, as semi-ovate, 
round, lunate, elongated, &c. Asaspecific 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 eee. 
Lwunuie. 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, 5 0. 
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 3, ¢ 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- 
