GASTROPODS 



345 



a large terminal mucus -pore, and there are other anatomical differences. The 

 single British species is here figured. Unlike Vitrina, the snails known as Zonites 

 and Vitrea, together with some allied genera, are wholly retractile within their 

 shells. There are several British species of Vitrea, and their identification from 

 the shells alone is a matter of considerable difficulty. In their habits they closely 

 resemble Vitrina, being carnivorous rather than vegetarian. They greedily devour 

 any kind of animal food, even in a putrid condition, and are also said to prey 

 upon some of the larger snails. They are generally found hidden away under 

 stones, dead leaves, or moss, and some of them emit a strong smell, like garlic, which 

 is perceivable at a distance of some feet. 



The Snail Tribe, — Family Helicid^e. 



Like the Limacidce, this family includes forms with or without an external 

 shell. In nearly all the genera the cutting-jaw of the animal is more or less 

 ridged, and not smooth as in Limax. The principal distinguishing character occurs 

 in the structure of the radula, which is composed of many rows of very numerous 

 similar square-based teeth, arranged so regularly as to have a tesselated appearance. 

 It is very broad, and the number of teeth in a row, although usually less, is some- 

 times as great or even greater than the number of rows. This family includes an 

 enormous number of species from all parts of the globe. These occur everywhere 

 and in all climates ; in dense forests, on the top of grassy downs, in valleys, fields, 

 lanes, in the arid desert, and at an elevation of some ten thousand feet both in the 

 Old and New World. 



To an ordinary observer, the members of the genus Arion (sometimes placed 

 in a separate family, Arionida) are merely slugs. Externally the resemblance is 



black sluo, Anon empi/ricorum (nat. sizej. 



very close, but the different position of the respiratory orifice, and the presence of 

 a mucus-pore at the end of the foot, readily separates this genus from Limax. 

 Besides these differences, the radula is of a different type. In Limax the breathing- 

 hole is situated near the hinder end of the shield, whereas in Avion it is much 



