6 PARTS OF BIVALVES EXPLAINED. ' 
Cuiass SEcoND.— CoNCHIFERA OR BIvALVEs, 
Or shells which consist of two parts or valves, These 
are connected by a cartilaginous process. In some species 
both valves are of equal size and form, while others are 
very dissimilar. In the genus Chiton, the species have 
generally eight valves placed transversely on the back of 
the animal, and inserted by their edges into a cartilaginous 
substance which surrounds them. They rank with univalves. 
Equilateral shells are those whose sides are alike, as 
exemplified in the shells of the genus Pecten. Fig. 20. 
Inequilaterai shells are those bivalves which have dis- 
similar sides, Plate VII. figs. 15, 16, 20, &c. 
Anterior slope or side is that side of the valve in which 
the ligament is situated. In looking at the anterior slope 
in front, the beaks of the shell retire from the observer. 
Posterior slope or side is that part of the shell opposite 
the anterior side ; or when viewed in front, the beaks point 
to the observer 
Umbo or Beak is the superior portion or point situate 
above the hinge, and which has been termed the apex. 
This part is the nucleus, from which the enlargement of 
the valves takes place, commencing in a small concentric 
circle, and continuing until the completion of the shell. 
Fig. 22. a., 20. b.,and 23.6. In some species the wmbones 
are flat and hardly visible, while in others they are long, 
prominent, and curved, as in plate VII. figs. 24 and 26. 
Hinge, the portion of the shell situate below the wmbones 
or beaks, and at which part the valves are united. Some 
hinges are simple, while others are complex, and are fur- 
nished with teeth, which fit into eavities in the opposite 
valve. Fig. 22. 3. 
Ligament, a cartilaginous process, by which the valves 
are united, and kept in their relative positions. It is always 
situate near the hinge, and may be either in front or rear of 
the umbo. 
Teeth of the Hinge, processes which are situate in the 
hinge of many bivalves. These are extremely variable in 
form and number, and are of much use in the generic and 
specific distinctions of bivalves. In some species they are 
large and single, (fig. 25. a.) while in others they are nume- 
rous and small, (fig. 23. a.) and, for the most part, they 
