GASTROPODS. 



349 



figured on page 344 is found all over Europe, and ranges even as far as Afghanistan, 

 The character of the radula is shown in the figure on the next page. 



The family Oncidiidce includes about fifty shell-less air-breathing molluscs., 

 somewhat slug-like in general appearance, but provided with a thick mantle covering 

 the whole dorsal region. This is frequently more or less tubercular, some of the 

 tubercles being furnished with eyes which, simple as they are in structure, are 

 identical in type with those of the Vertebrata. These snails live on the seashore 

 or in brackish marshes, eating nothing but sand, but, of course, only digesting the 

 nutritious organic particles contained in it. Professor Semper has given an 

 account of their habits, and how they are pursued by certain fishes (Perioph- 

 thalmus), which come ashore after them. One species, 0. celticum, is found in 

 Cornwall and South Devon, others occur in America and the Pacific. 

 Basommato- The numerous forms belonging to this group, comprising the rest 



phora. of the Pulmonata, differ from those already indicated, in having the 

 eyes situated at the base of the 

 tentacles, as in the marine whelks \ \ Vjhlf/) 

 and periwinkles, instead of at the 

 tips. An external shell is always 

 present, and capable of containing 

 the entire animal. The members 

 of the family Auriculidcb mostly 

 Eariet Shells, inhabit salt or brack- 

 Auricuiidse. j^ marshes, and were 

 formerly regarded as marine mol- 

 luscs. The shells generally are 

 rather solid, of different forms, but 

 usually with narrow apertures, more or less contracted by teeth. The internal 

 septa between the whorls of, the spire are often absorbed or dissolved, excepting 



that between the last and penulti- 

 mate volutions. Some species of the 

 typical Auricula have large heavy 

 shells, and are met with in mud- 

 banks and in swamps, in the Indian 

 Archipelago. Those belonging to 

 Pythia have oval, compressed shells, 

 with toothed apertures, and occur 

 in great numbers in most places in 

 woods near the sea, are wholly ter- 

 restrial in their habits, and feed on 

 decayed vegetation. One member of 

 this family (Carychium minimum) 

 is found in Great Britain, and on the Continent. It is an extremely small 

 shell, less than the tenth of an inch in length, transparent, glossy, having 

 three teeth -like projections within the aperture. It hides away at the roots 

 of grass, among moss, dead leaves, or under stones or dead wood, in damp 

 situations. 



Pythia scarabceus (nat. size). 



earlet shell, Auricula judoe (nat. size 



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