50 LAND AND FRESH-WATER MOLLUSKS. 
The length of the lingual ribbon is short in 
Paludina, but is of varying length for different 
species; in the marine limpet it is longer than 
the whole animal. The teeth are distributed in 
straight longitudinal rows, and in transverse 
rows, which are variously curved, angular, or 
rarely straight. The number of the teeth in the 
transverse row is nearly constant for the same 
species, and the number of rows is exceedingly 
variable in different species; longitudinally the 
teeth are usually arranged in a triple series, and 
each transverse row is but a repetition of the 
rest. The central area is called the rachis , and 
the teeth form usually a single series ; the lateral 
areas are called the pleurae, the teeth on which 
are termed uncini , and usually are extremely 
numerous. The term laterals is employed in a 
restricted sense to designate a series of teeth 
intermediate between the rachidian and the 
uncini . 
The lingual ribbon of Paludina vivipara con- 
Fig. 7-—Teeth of Paludina vivipara (Loven). 
sists of a few transverse rows, each composed of 
