CLASSIFICATION. XXIX 



acters and unusual forms. In the Carboniferous numerous very 

 large species occur, some of them surpassing any of the recent forms. 

 These belong mostly to the subgenus Plagioglypta, a group which 

 became extinct before the close of the Mesozoic. 



Families of Scaphopoda. 



I. Width of the median tooth of the radula double its height. 

 Foot with an encircling epipodial sheath which is discontinuous, 

 interrupted on the side next to the head. Shell greatest in di- 

 ameter at the oral opening, DENTALIIDJi;, p. xxix. 

 II. Width of median tooth much less than double its length, gener- 

 ally less than the length. Foot vermiform, capable of expan- 

 sion into a terminal or subterrainal rosette-like disk, not inter- 

 rupted dorsally. Shell generally smooth, often inflated, 



SIPHONODENTALIID^, p. 130. 



Family DENTALIID^. 



Characters given above. A single genus is generally recognized. 

 Genus DENTALIUM Linne, 1758. 



Dentalium L., Syst. Nat. (10), p. 785, and of most subsequent au- 

 thors. — Dentalites Schlotheim, Die Petrefactenkunde, p. 93 (1820), 

 and of Brongniart, Diet. Classique d'Hist. Nat., v, p. 419 (1824), 

 DeFrance, Diet. Sc. Nat., xiii, p. 73 (1819). — Entalites Walch., 

 Naturg. Verstein., ii, p. 276 (1768). — Dentalis Llwyd., 1698, Lang, 

 1722, Klein, 1731, and some other pre-Linnean authors. — Dentalites 

 ScHROETER, Litholog. Lex., i, p. 405 (1779). Syringites, Canalites, 

 Tubulus and Tubulites of some early authors (see Hermaunsen). 



The genus Dentalium, as herein understood, is co-extensive with 

 the family Dentaliidce. The synonyms cited above are all obsolete, 

 and none of them available for subgeneric groups. 



Key to subgenera of Dentalium. 



In using the following key, much difficulty will be found in dis- 

 tinguishing Dentalium restricted, ^4?ito/is and Fissidentalium. These 

 groups are closely allied, and while natural assemblies, we would 

 scarcely have segregated them as subgenera had they not already 

 been named. Moreover, certain species of Antalis and Graptacme, 

 when old, have lost the characteristic longitudinal sculpture, and 

 become not readily distinguishable from Lievidentalium. The sub- 

 genus Comjjressidens (which may prove to belong to Siphonoderital- 



