24 The Shell-Collector’s Handbook. 
known as the hinge-line. Near to this line, in some 
species as Spherium corneum, there are teeth which 
fit into corresponding hollows in the opposite valve. 
Those directly under the umbo are called cardinal 
teeth, those on either side lateral teeth. The length of 
the shell is measured from its anterior to its posterior 
margin, and the breadth from its dorsal to its ventral 
margin. 
Microscopical examination of the shell shows three 
layers, the periostracum or outer layer, the prismatic 
or middle layer, and the nacreous or inner layer. The 
periostracum is thin and horny; the prismatic layer 
is composed of polygonal prisms placed side by side in 
a pallisade manner, and the nacreous layer consists of 
super-imposed calcareous lamine. 
Muscuntar System.—The anterior and posterior 
adductors run from one valve of the shell to the other. 
The anterior and posterior retractors of the foot are 
situated near these, and pass from their origins to 
their insertion into the foot. The protractor muscle 
arises from the protractor impression, and spreads out 
in a fan-shaped manner over the upper part of the 
foot. Several small muscles—the pallial muscles— 
attach the mantle to the pallial line, while some 
muscles resembling these, and known as the lesser 
retractors, arise in front of the umbo, and spread over 
the surface of the digestive gland. 
THe Mantie and Mantie-Cavity.—The mantle is 
divided into two lobes, a right and a left, which are 
continuous with each other on the dorsal side of the 
animal. Between the two lobes is the mantle- or 
pallial-cavity containing the gills, foot, labial palps, 
