8 EOCENE MOLLUSCA. 
terms with which the conchologist, rather than the malacologist, is most familiar; I 
therefore introduce a figure with a formula of explanation, making as little alteration 
in existing terms as possible. 
Lr LE 
ime NN 


/ ; | i 
/ 0.a.m : ip aan 
do a lise Ne eee \ ‘vhs 


Pe rr opiten gi mn po NS 7 
\ . j 
u. Umbo. c. p. Cartilage pit ’ 
u. r. Umbonal region. l. f. Ligamental fulernm So cess 
p. 7. Pedal region. p.m. Pedal muscle. 
s.7. Siphonal region. o. a.m. Oral adductor muscle. 
i. 7. Lunule region. a. a.m. Anal adductor muscle. 
ec. r. Corslet region. p. @. Palleal impression. 
d.m. Dorsal margin. s. s. Siphonal sinus. 
v. m. Ventral margin. ec. ¢. Cardinal teeth. 
p. l,m. Pedilateral margin. 1. ¢. Lateral teeth. 
s. 7, m. Siphonilateral margin. d.v. Depth of yalve. 
A considerable difficulty exists im 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 are locomotive ; 
therefore the height should be from that base to the vertex or ‘mbo, 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 are 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 
