XXXVIil DESCRIPTIVE REMARKS. 
Fic. 2.—A BIvALVE SHELL. 
Class CONCHIFERA, Cytherea chione (recent). 
Original reduced to two-thirds of natural size. 
Interior of left valve. Exterior of ditto. 
WHD 
Both valves united. 
A.—Interior of left valve. B.— Exterior of left valve. 
uw. Umbo. aw. Umbo. 
d. m. Dorsal margin. 7, Lunule. 
». m. Ventral margin. d. m. Dorsal margin. 
a. s. Anterior side. ». m. Ventral margin. 
p. 8. Posterior side. a. s. Anterior side. 
ce. t. Cardinal teeth. p- 8. Posterior side. 
1. ¢. Lateral ee c. 1. Concentric lines of growth. 
h. l. Hinge ligament. ; 
ih ee 2 C.—Right and left valves united. 
a.m. i. Anteriormuscular impression. «w. Umbo. 
p.™. i. Posterior __,, x 7. Lunule. 
p. %. Pallial impression. h. l. Hinge ligament, 
s. 8. Siphonal sinus. 1. 7. Line of junction of valves. 
The dotted lines on figure B, marked ‘length’ and ‘‘height,”’ 
also that on fig. C, marked ‘‘ depth,” are intended to show the direc- 
tions in which measurements should be taken in describing fossil shells. 
Tt would be desirable if a uniform system of measurement were 
adopted, by lines, reckoning 12 to the inch. 
Bivalve shells are all aquatic, and, excepting a few fresh water 
genera, all marine. 
Fossil Bivalves are frequent in all sedimentary rocks; in the oldest, 
or Paleozoic strata, they are few, but gradually increase through the 
secondary and tertiary, attaining their maximum development in the 
present seas. 
