42 THE CHICAGO ACADEMY OF SCIENCES. 
pressed against the top of the mouth. The horny jaws serve to 
cut the food into small pieces for the radula to act upon. The 
chitinous radula is constantly growing forward from the core or 
diverticulum (3) in the same manner that the human finger nail 
grows upon its bed, and ceases to grow as the anterior end is 
reached. As fast as the front end wears out from use it is 
replaced by fresh material from the core. A layer of cells lines 
the under surface of the radula which carries the latter forward. 
The radula is a belt of chitinous, transparent, yellowish or 
colorless material, and its upper surface is beset with a large 
number of siliceous teeth, arranged in parallel rows (Fig. 8). 
Two complete rows of teeth on the left side of a membrane of PoLycyra 
TRIDENTATA Say (Binney & Bland, L. and F-W. Sh., fig. 220). 
The portion of the radula nearest the mouth (anterior) is the 
most developed, and at the same time the most worn, while the 
posterior portion, or that part nearest the radula sac, is the least 
developed. The radula is usually divided into five longitudinal 
rows, each row differing from the one next toit. Thus we have a 
central row, on each side of this a lateral row, and on each side 
of this a marginal row (Fig. 9). Each tooth in each row is 
Fie? 9. 
Diagram of teeth on the molluscan radula. a. median; b, lateral; c, 
uncinal or marginal. (Original.) 1., base of attachment to radula; 2., re- 
flected portion; 3., side cusps; 4., median cusps; 5., cutting points of side 
cusps; 6., cutting point of median cusps. 
made up of a number of different parts which, in their diversity, 
‘serve to distinguish the different groups of mollusks. Thus the 
central tooth has a strong, quadrate base of attachment (1) to 
