UNIVALVES. oe 
a central oval tooth (a), slightly hooked and 
denticulated ; and three nearly similar wncini 
(b, c, d), which are oblong and toothed on the 
upper sides: the number of teeth in each trans- 
verse row is therefore seven. 
The number and arrangement of the teeth are 
capable of easy representation by a numerical 
formula. Thus, 3.1. 3 represents the system 
in Paludina, signifying that each transverse row 
consists of one median or rachidian tooth, flanked 
on each side by three wneini. 
In Vitrina the general formula is «. 1. w; 
where oo represents 37; and as there are 100 
rows, the lingual teeth of Vitrina are 7,500; the 
formula will now stand ***-87 — 7,500. In the 
great slug, Limax maximus, there are 28,800 
teeth, distributed in 160 rows of 180 teeth in 
each. The number of teeth has no relation to 
the size of the animal; thus, Helicella cellaria 
possesses 1,330, while H. nitidula, less in size, 
has nearly three times that number. 
The above dental formula will be employed to 
indicate the number of teeth im each generic 
group or species, as the case may be. 
The teeth of the fresh-water Gasteropods are 
characterized by their fewness, whilst those of 
air-breathing snails are remarkable for their 
extraordinary number. 
The digestive apparatus further consists of a 
E2 
