32 



SHELL GALLERY. 



rest of the Limneeidce, feed on freshwater algse, confervas, and 

 decayed vegetable matter. 



The " Limpet-Snails " (Siphonariidce) seem at first sight to be 

 out of place among the Snails and Slugs, and more nearly allied 

 to the Rock-Limpets ; but the character of the tongue (odonto- 

 phore) and the closed respiratory cavity indicate a close relation- 

 ship with the present group. 



The shells of Siphonaria may be known from Limpets by a 

 slight bulging on one side, caused by a radiating groove which 

 interrupts the muscle of attachment. They are marine, and are 

 found on rocks between tide-marks, chiefly in tropical countries. 



[Case 

 17 D.l 



Fig. 21. 



SCAPHOPODA*. 



The " Tooth-shells " (Dentaliidce) form a distinct group, the 

 shells of which are very unlike those of 

 any other mollusk, but closely resembling 

 the shelly tubes constructed by certain 

 kinds of marine worms. The Dentalia 

 have neither eyes nor tentacles, or a dis- 

 tinct head like Gastropods ; their organs 

 of circulation and respiration are of a 

 rudimentary kind, and they have no heart. 

 Their foot is adapted for burrowing in 

 sand, in which they live and obtain their 

 food, which consists of Foraminifera 

 and minute Bivalves. One species, 

 Dentalium pretiosum, found on the 

 shores of North-west America, was until 

 recently used as money by the Indians. 



From the Greek : scaphe, a small boat, 



British Tooth-shell 

 (Dentalium tarentinum). 



arid pons a foot — the foot of some Scaphopods a . The shell, b. The animal. 



being somewhat pointed like the prow of a 

 vessel. 



removed from its shell 

 /, the foot. 



