8 BULLETIN 111, UNITED STATES NATIONAL MUSEUM. 



Genus DENTALIUM Linnaeus, 1758. 



1758. Dentalium Linnaeus, Syst. Nat., ed. 10, p. 785. 



Type. — D. elepliantinum Linnaeus (Amboyna). 



Shell an elongate tube open at both ends, increasing in diameter 

 from the apex to the aperture, the section of maximum diameter 

 being coincident with the peristome; almost straight to strongly 

 curved; usually sculptured with longitudinal (lengthwise as from tip 

 to aperture) riblets or engraved lines, especially near the tip or on 

 the posterior portion. The sculptural features may vary in degree 

 from faint indications to strong heavy ribs, and these may vary in 

 number from 6 to 60 or more. The original number of ribs or riblets 

 in the apical portion are often increased later by the addition of others 

 intercalated between them, so that as the animal grows its shell in- 

 creases in the number of its ribs. Frequently all sculptural features 

 present in the posterior or middle portion of the shell disappear in 

 the anterior portion, the senile stage showing a smooth surface. The 

 sculpture is sometimes further modified by fine transverse lirae occupy- 

 ing the spaces between the longitudinal ribs or even crossing them. 

 Again, the intercostal surface may present a fine reticulated plan of 

 excessively minute sculptural elements. In some groups there are no 

 sculptural features whatever. The embryonic portion of the apex is 

 very minute and fragile, and is always lost, save rarely, in very young 

 specimens. The apical section remaining may be round or angular 

 in section, in which latter case the angles become the beginnings of 

 the primary ribs or riblets. The apical opening is usually modified 

 by a slit or notch of varying width or depth and variously placed in 

 difi'erent groups; or the opening may be simple without either notch 

 or slit. The shell varies in size from minute needle-like forms to those 

 of 4 or 5 inches in length, in tliickness from fragile to heavy and sohd; 

 in texture from soft and chalky to hard porcellanous or glassy; in 

 color from occasional greenish, reddish, or yellowish species to pure 

 white, the latter greatly predominating. The' shells may be trans- 

 parent, translucent to opaque, dull lusterless to the most highly 

 polished and glistening surface. 



The animal is shaped like the shell it occupies — elongate, wrapped 

 in a mantle open at both ends. From the larger end a pointed 

 cylindrical foot may be protruded or almost wholly withdrawn. It 

 is capable of more or less expansion at the end. There are no ten- 

 tacles or eyes. Just back of the mouth is a cluster of thread-like 

 appendages enlarged into spoon-shaped terminals (cap taeniae). 

 The exact function of these seems not to be definitely understood, 

 but they are supposed to catch and hold the food, consisting largely 

 of foraminifera and other minute organisms. The liver is two- 

 lobed, the gonad simple; the heart consists of one ventricle, and 

 there are no gills. The excretory openings are in the forward part 



