40 COMPAEATIVE DENTAL ANATOMY 



The highest type of these molluscan teeth is called 

 the Toxoglossal, or arrow-tooth, from its narrow, 

 round form, often barbed, sometimes hollow to 

 inject poison. They have but two rows, — the 

 middle or rachidian being absent. The Bachi- 

 glossa have only the middle row, or rachidian 

 teeth. The teeth are small and varied in form, 

 and prettily denticulated on the cutting edge. 

 They are few in number. The Ptenoglossa, 

 feather-toothed, is a small group. They lack the 

 middle rows, but have numerous small teeth on 

 the side of the tongue. The Docoglossa, chevron- 

 toothed, is a large group, and presents consider- 

 able variation among its members as to the pres- 

 ence or absence of the different rows of teeth. 

 The Eaphidoglossa, needle-toothed, have large 

 numbers of uncini teeth, the other rows varying 

 in different groups. They usually have a well- 

 developed mandible, or jaw, which is hinged in 

 the middle. The Taenioglossa, bent-toothed, in- 

 cludes the greater number of fresh-water snails. 

 The teeth vary in number and are often absent 

 entirely. The common Helix, or air-breathing 

 snails, often present a pavement-like form and 

 arrangement of the teeth, which are often of a 

 very pretty pattern, or, again, are a mere hard- 

 ened mass. The lingual ribbon, beset with such 

 teeth, is well adapted for filing off or rasping food 

 and drawing it backward into the mouth. Be- 



