44 STUDIES IN GEOLOGY, No. 3 



Shell large, conical, ovate, two-thirds as wide as long, 

 thick. Spire rather short and acuminate. Sutures indis- 

 tinct. Body whorl regularly conical, very convex, broadest 

 one- fourth from top, forming a well rounded shoulder, and 

 tapering rapidly from this point to end of spire. Aperture 

 linear and narrow. Outer lip with the margin thick and 

 broad. Columellar lip with four nearly equal, well developed 

 plaits ; the two upper more widely separated than the lower 

 ones." Nelson, 1870. Height, 28; diameter, 18 mm. 



This species is very close to M. coniformis Sowerby, 36 

 from Antillean Miocene horizons, differing only in lacking 

 the slight depression behind the posterior canal which is 

 characteristic at least of the Bowden specimens of coni- 

 formis, in being distinctly larger and heavier, with a deeper 

 posterior canal. The body whorl of incrassata tapers more 

 sharply on the ventral side. The two are very close, and if 

 incrassata were not so much larger than coniformis it might 

 readily be regarded as a variety of that species. 



It is also very close to M. aurora Ball, 87 from the Chipola 

 beds of Florida. The posterior canal of incrassata is not as 

 deep, the spire is somewhat lower, and there are no denticu- 

 lations on the inner surface of the outer lip as in aurora. 

 N. mavensis Maury, 33 from the Santo Domlngan Miocene, 

 is similar, differing in having heavier plaits on the pillar, 

 and in that the body whorl is not as sharply tapered as in 

 incrassata, 



Zorritos formation. Zorritos. 



36 Quart. Jour. Geol. Soc. Lond., vol. 6, p. 45, 1850. 



"Trans. Wag. Inst, vol. 3, pt. i, p. 51, pi. 6, fig. 4a, 1890. 



88 Bull. Amer. Pal., vol. 3, No. 29, p. 71, pi. n, fig. 7, 1917. 



