BIVALVIA. 3 



merely, in consequence of a difference in the length of the tubes or depth of the 



sinus.* 



Some Malacologists seem disposed almost entirely to reject the shell, as unworthy 

 of consideration in a Zoological arrangement, viewing it in the light of an inert or 

 inorganic mass, unconnected with the animal, or at least merely formed by and used 

 as a protection to its more vital parts, and have based their superstructure upon the 

 mantle itself, and upon the difference in length of its siphonal tubes. In the ' History 

 of British Animals,' by Dr. Fleming, published in 1828, the Bivalvia were separated 

 into two sections, called Siphonida and Asiphonida, a division subsequently 

 adopted by some continental authors under the denominations Sinupalealia and 

 Integropaleali a, as founded upon a portion of the animal more highly organised than 

 its dermal covering, and, consequently, supposed to give a more scientific basis to its 

 classification. Investigations by the microscope have shown a high degree of organi- 

 sation, and the possession of a considerable amount of vitality in the shell, essential to 

 the existence, depending upon, and modified by the exigencies of the animal ; and in 

 this outer coating of the mantle there is preserved a relationship apparently more con- 

 stant than is exhibited by its fleshy interior ; and whatever other organs, in the more 

 vital parts may be supposed to furnish a basis for Ordinal division, it is very doubtful 

 if the form of the mantle alone will be sufficient. 



The number and position of those parts of the hinge called teeth are essential 

 distinctions, as there is a permanence of form in the dentition of all genera, although, 

 in a few instances, these characters which are prominent and distinctive in some 

 species, will be diminished and become nearly obsolete in others ; but they do not 

 vary in form or position in the same genus. f That portion of the hinge called the 

 ligament, performs an important office in the animal economy, as it is by this the 

 valves are bound together, and kept in their true position. This uniting and elastic 

 substance is called cartilage, when it is placed within the edges of the valves, and is 

 consequently compressed when they are closed, and by its tendency to expand at the 

 relaxation of the adductor muscle or muscles, assists in the separation of the shells at 

 the ventral margins : that portion which is external, is called ligament, and is generally 

 placed on a prominent fulcrum, or projecting portion of the shell, and by its elasticity 

 or contraction draws back and opens the valves when the opposing power of the 

 adductors is relaxed ; although this substance is of a cartilaginous nature, and con- 

 tains but a small portion of lime, and is consequently not often preserved in a fossil 

 state, its position is always indicated where it has been, either by a pit or depression 

 for its reception, or by the fulcrum to which it was attached. In the smaller portion 

 of the Bivalvia, the animal is furnished with only one adductor muscle, and constitutes 



* Great differences also exist between the mantles of some of the members of the Leptonidse. 



f There is a slight exception to this rule in the hinge of the Polyodonts, Leda, Pectunculus, #-c, where 

 the number of teeth will vary, even in individuals of the same species at different periods of existence, 

 but their general character is not altered. 



