Maryland Geological Survey 507 



strongly and evenly rounded medially and basally; shell not preserved 

 but traces of a very fine and crowded spiral lineation still discernible on 

 the cast near the aperture ; aperture holostomous, subcircular in outline ; 

 body whorl rounding evenly into the rather large and very deep umbili- 

 cal pit. 



Dimensions. — Altitude 4.5 mm., maximum diameter 4.2 mm. 



This species is separated from its close kin, M. abyssina (Gabb), by its 

 more elevated spire and consequently smaller apical angle and the larger 

 number of constituent whorls. 



Occurrence. — Monmouth Formation. Brightseat, Prince George's 

 County. 



Collection. — Maryland Geological Survey. 



Class SCAPHOPODA 



Family DENTALIIDAE 



Genus DENTALIUM Linne 



[Systema Naturae, ed. x, 1758, p. 785] 



Type. — Dentalium eleplianticum Linne. 



Shell solid, tubular, tusk-shaped, open at both ends; external surface 

 smooth, longitudinally sulcate, or annulate: anterior orifice simple, not 

 contracted. 



" The researches of Sars have shown that the arched side of the shell 

 is to be regarded as ventral and the concave side dorsal, but to avoid con- 

 fusion in comparisons I have not corrected the terms in general use, 

 which are the exact opposite of these. The posterior end of the shell in 

 Dentalium may be circular or ovate and evenly truncate, oblique, undu- 

 lated, fissured in the median line, or with slits, lateral or ventral, or both. 

 " The most usual form is that which is evenly transversely truncate ; 

 the next most common style is one in which there is a dorsal wave, or 

 sulcus, or even a narrow slit of some length. The other combinations 

 are less common. In a wide sense these variations depend upon the form 

 of the mantle-edge or internal lining of the shell, which is an extremely 



Etymology: A derivative of dens, a tooth. 



