8 EOCENE MOLLUSCA. 



terms with which the eonchologist, rather than the malaeologist, is most familiar; I 

 Therefore introduce a figure with a formula of explanation, making as little alteration 

 in existing terms as possible. 



/"• / iU 



s.l.m. 



u. Umbo. 



u. r. Uinbonal region. 



p. r. Pedal region. 



s. r. Siphonal region. 



/. r. Lunule region. 



e. r. Corslet region. 



d. m. Dorsal margin. 



v. m. Ventral margin. 



p. I. m. Pedilateral margin. 



s. f. in. Siphonilateral margin. 



c. p. Cartilage pit 



/. f. Ligamental fulcrum 



p. in. Pedal muscle. 



o. a. m. Oral adductor muscle. 



a. a. m. Anal adductor muscle. 



j). i. Palleal impression. 



s. s. Siphonal sinus. 



c. t. Cardinal teeth. 

 /. t. Lateral teeth. 



d. r. Depth of valve. 



> connexui 



A considerable difficulty exists in the mode of estimating the dimensions of a 

 Bivalve, from the want of accordance amongst conchologists in denominating the 

 different parts of the shell, the length being sometimes considered to extend from the 

 umbo to the ventral margin, the breadth from the pedilateral to the siphonilateral 

 margin, and the most tumid portion of the closed valves as forming its height. At other 

 times it is the reverse of this. In my monographs I have adopted the latter method, 

 conceiving it to be the more natural one. The base on which the animal stands is the 

 ventral margin, and on which it is moved by the foot in all those which arc locomotive ; 

 therefore the height should be from that base to the vertex or umbo, and the length 

 consequently should be taken in an opposite direction, while the tumidity of the valves 

 may be considered the depth, and the substance of the shell its thickness. 



In the Dimyaria, which arc generally transverse shells, and have a greater diameter 

 from the pedal to the siphonal region, there is no difficulty in thus considering these 

 various dimensions ; but with the orbicular shells, or with those whose elongation is from 



