724 GEOLOGICAL SURVEY OF IOWA. 



Dentalium obsoletum ( n. s.). 



PLATE XXIX. FIG. 16, 17 & 17 a. 



SHELL thick, terete, slender, very gradually tapering, 

 slightly arcuate : section at the smaller extremity circular, 

 slightly compressed below (perhaps from accident). 



SURFACE marked by longitudinal slightly undulating striae, 

 which become somewhat obsolete on the lower half of the 

 shell, and more distinctly crossed by transverse striae of 

 growth. 



This shell is marked near the apex by regular longitudinal striae, which are uni- 

 form and conspicuous on the middle and lower parts, and on the outer side of the 

 curve are partially or entirely obsolete, while at the same time the concentric striae 

 become more conspicuous. The fragments figured show the shell to have been extreme- 

 ly elongated. 



This species, in its longitudinal striae, bears some resemblance to D. ornatum of 

 DE KONINCK, but is more slender and less arcuate, while the striae are coarser. 



Fig. 16. A fragment near the base of the shell. 



Fig. 17. A more slender fragment from the upper part of the shell. 



Fig. 17 a. Enlargement of the longitudinal striae. 



Geological formation. In the Coal measures. 



