SCAPHOPODA ^91 



retained tliem among the snails, largely because of the radula and jaw, 

 and in 18(i2 the latter author introduced the name Scaphopoda, but 

 for a long time the opinion prevailed that the true position of the group 

 was between snails and bivalves, and that they represented the 

 common ancestors of both. At present the latter view is not generally 

 held, and the systematic position of the Scaphopoda is still in dispute. 

 There can be do doubt, however, that they are vei-y primitive mollusks 

 and so aberrant in structure that they should be placed in a Class by 

 themselves. 



The Scaphopoda are all marine animals, living in clean sand in 

 various depths from shallow water to 2,600 fathoms; they are found in 

 all seas except the polar. About 200 species are known, grouped in two 

 families; about 350 fossil species are known. 



Key to the families of Scaphopoda: 



«i Foot trilobate 1. Dentaliidae 



tta Foot elongate, vermiform 2. Siphonodentaliidae 



Family 1. DENTALIIDAE. 



Foot with 2 lateral epipodial lobes, making it trilobate; oral palps 

 present; lateral teeth of the radula dentate; width of the median tooth 

 of radula double its height : 6 genera and 50 species, most of which are 

 found in deep water. 



Dentalium L. With the characters of the family: 9 

 subgenera and numerous species. 



D. entale L. (Fig. 754). Shell not much curved, 

 glossy and ivory-like, annulated, without longitudinal stri- 

 ations except at the smaller end; 50 mm. long; 5 mm. nSialium 

 wide : Cape Cod to the Arctic Ocean, in 3 to 1,750 fathoms; (Kingsley) 

 common; Europe. 



D. pretiosum Sowerby. Like the above, but larger, slenderer, and 

 whiter; length up to 55 mm.; diameter 5 mm. : Pacific coast from Sitka to 

 Lower California. 



D. occidentale Stimpson {D. dentale Gould). Shell curved like an 

 elephant's tusk, glossy, yellowish-white, with about 20 longitudinal stri- 

 ations, and about 25 mm. long and 8 mm. in diameter: New England 

 coast north of Cape Cod, in from 8 fathoms to deep water. 



Family 2. SIPHONODENTALIIDAE. 



Foot vermiform, capable of expansion into a terminal disc; width of 

 median tooth much less than double its length: 5 genera and about 60 

 species, most of which are found in deep water. 



