108 
DENTALIUM. 
(Plate XXII. Fig. 1.) 
Shell univalve, straight, or nearly so, subconical, 
tubular, not chambered, open at both ends. 
The simple construction of the shells in this 
genus, and the paucity of species, render it unne- 
cessary to offer any extended remarks on their 
distinctive character. They are all more or less, 
as their name expresses, like teeth or tusks, and 
are completely separated from the other tubular 
families, by being entirely without contortion, 
though somewhat curved. 
They are usually discovered partly buried in the 
sand ; and the animal, which has by some natu- 
ralists been supposed perfectly free and unattached 
to his habitation, is seen to shrink deeply into it 
for protection from impending danger. The opi- 
nion, that the Terebella possesses the power of dis- 
engaging himself from his shell, has arisen pro- 
bably from the circumstance of there being no 
apex, hinge, or visible depression to which the 
