Maryland Geological Survey 509 



has a dorsal slit, very narrow and prolonged in some cases. But it often 

 happens that erosion, especially in specimens from deep water, modifies 

 and sometimes simulates such slits, introducing them where normally 

 they should not be, or lengthening them abnormally. There seems to be a 

 peculiarity of some kind in the external prismatic layer of Dentalium, 

 which lends itself to the propagation of erosion in longitudinal lines very 

 much more effectively than at right angles to such lines. Hence we see 

 specimens of a species, normally provided with a short slit, exhibiting an 

 enormously long slit, or, starting at some little defect in the posterior 

 margin, a narrow line of erosion, simulating a slit, may run a long dis- 

 tance up the shell. These abnormalities may usually be discriminated by 

 comparison with numerous specimens of the same species. In cases where 

 the student has only one or two specimens, he should refrain from putting 

 reliance on characters which may be abnormal as a basis for describing 

 new forms or for discriminating old ones. 



" It may also be added that it rarely happens that smooth species do not 

 show at least a little sculpture near the posterior end, or that sculptured 

 ones do not show a modification of the sculpture toward the anterior end. 

 Hence a broken fragment from either part of the shell can hardly be 

 relied upon to give differential characters for the species as a whole. In 

 the same species, among the sculptured ones a good deal of variation in the 

 strength of the sculpture between different specimens is extremely com- 

 mon and should always be allowed for." — W. H. Dall, 1892. 1 



This genus is one of the most ancient of all the molluscan phylum. 

 There is evidence of it early in the mid-Pala3ozoic, and in the late Meso- 

 zoic it was abundantly represented. The recent species number about one 

 hundred and fifty. The Larger forms are many of them abysmal. The 

 animal lives, for the most part, buried head downward in the sand or mud, 

 with the mantle spread to function as a gill, and the posterior end of the 

 test extending obliquely upward into the clear water. 



1 Trans. Wagner Free Inst. Sci., Phila., vol. iii, pt. ii, pp. 436-43S. 

 33 



