142 UNIVALVES. OENTALIUM. 



Neritoidea Chambered. Aculeata Spiny-ribbed. 



Porcellana Brown-spot. Goreensis Sandal. 



Fovnicata Slipper. Crepidula Transparent- white. 



DENTALIUM TOOTH OR TUSK-SHELL. 



Animal a Terebella : Shell univalve, tubular , straight, or 



slightly curved, with an undivided cavity open at both ends. 



OF this singular genus there are hut fifteen species; 

 they are very similar in form, and resemble an elephant's 

 tusk in miniature. 



They differ principally in magnitude, and in the number 

 of ribs and grooves with which some of the species are 

 marked; a trifling distinction may also be made with re- 

 gard to the degree of curvature which many of them pos- 

 sess. 



One of the largest and most valuable species is the D. 

 elephantinum ; it is often three or four inches long, slight- 

 ly curved, and has generally ten strong, elevated ribs, 

 which are encircled by dark green bands on a greenish 

 ground; it inhabits the Indian and European seas. 



The D. rectum, though nearly allied to the D. elephan- 

 tinum, is a straight shell, and therefore easily distinguish- 

 ed from it. It is also adorned with doubled or tripled 

 longitudinal striae, which at the same time are encircled 

 with annular ones. The D. fasciatum is a small species, 

 finely striated, and encircled with four or five brown bands 

 on a greyish ground. 



Some species are striated annularly : such are the D. po- 

 litum, (which is finely pointed, solid, and often of a tosy 

 or pinkish colour), and the D. eburneum. 



