ANATOMY OF THE LAMELLIBRANCHIATA. 127 



others, as the P. maximus, have the convex valve commonly 

 buried in the sand. These free species of Pecten swim and 

 leap by striking the water with their valves, closed by the 

 action of the voluminous adductor muscle. It is evident in 

 opening an oyster how powerful this muscle is ; in the Pec- 

 ten it is much more so. The principal organ of locomotion, 

 however, in these animals is the foot. The Monomyaria 

 have it little developed, some, as the oyster, having no trace 

 of it ; and in them it seems of little use as an organ of loco- 

 motion. When present in them it is of a cylindrical figure, 

 expanded at the extremity as in Lima, Pecten, &c. In the 

 Spondylus, from its terminal disk a filament depends, at the 

 extremity of which is a small oval body. In these genera a 

 long slender muscle arises from the upper part of the left 

 valve, and is inserted into this organ, bending it when in 

 action, up to the mouth. The byssus, according to Cuvier, 

 is present in the Perna and Malleus ; in one species of Lima 

 the author does not find it, though Blainville and Cuvier also, 

 correctly says it is present in another. It exists also in the 

 Avicula, Pinna, Lithodomus, Byssomya, &c. The foot, 

 which moulds it, receives several pairs of muscles, originating 

 from the valves, at different points, and inserted into its base. 

 In the Pecten and the other Monomyaria, there is but one 

 adductor muscle. In Avicula, Pinna, Mytilus, &c, another 

 is added at the anterior part of the shell ; in them, however, 

 yet small. In Lithodomus the anterior one is become equal 

 to the other ; in some species of Solen it is much the larger 

 of the two. These muscles pass directly from one valve to 

 the other, and are the antagonists to the force of the elastic 

 cartilage. The foot, in the Dimyaria, varies in its form, and 

 is occasionally very large. It has circular, longitudinal and 

 oblique fibres, and is attached to the valves by two or more 

 pairs of muscles as mentioned above. In Area it has a horny 

 substance at its lower part, analogous to the byssus of other 

 genera. In Nucula it is tentacular at its lower circumference. 

 In the JJnio it is large, oval, and slightly compressed laterally, 

 with anterior and posterior retractile muscles ; and there is 

 an orifice at its posterior extremity, by which the animal can 

 distend it with water ; as is the case in a greater degree in 

 others, as the Solen. In the Cyclas it is elongated, compress- 

 ed and blunt; in the Cardium round, and bent at a right 

 angle in the centre, and pointed at the extremity. In the 

 Mactra it is very long, large and lanceolate. It is broadly 

 lanceolate in Tellina, Psammobia, &c. ; larger and falciform in 

 Donax. It is securiform and rather expanded below in Pec- 

 tunculus ; of the same outline, but sharp inferiorly, in the 



