820 POLYPLACOPHORA AND SCAPHOPODA. 



of the tube. The fossil species of Dentalium may be confounded 

 with the tubes of Tubicolar Annelides, or a reverse mistake to this 

 may be made. Species of Dentalium have been described from the 

 Ordovician rocks of Russia by Eichwald, but the nature of these is 

 not absolutely certain. Undoubted forms of the genus appear in 

 the Devonian rocks, and a number of Carboniferous species are 

 known. In the Secondary and Tertiary rocks the genus is well 

 represented, the later forms being usually more ornate as regards 

 the characters of the surface than are the older types. 



In the genus Siphonodentalium (Gadus) the tubular shell may or 

 may not be contracted towards its anterior extremity, but is always 

 attenuated behind ; and the posterior aperture is incised or lobulated. 

 The surface is smooth or finely striated. The genus ranges from 

 the Cretaceous rocks to the present day. Lastly, in the Recent 

 and late Tertiary genus Cadulus, the tube is swollen in its middle, 

 and both its apertures are entire, the hinder one being crenulated. 



