720 GEOLOGICAL SURVEY OF IOWA. 



SURFACE very finely striate in direction of the lines of 

 growth. 



This species bears some resemblance to M. ventricosus, but is comparatively less 

 ventricose, and the last volution is proportionally less in length than in that species. 



The Genus PLECTOSTTLUS of CONRAD was founded upon a cast which very much 

 resembles the cast of this species; and should it prove identical, the name Macro- 

 ckeilus hildrethi will supersede the present one. 



Fig. 10. View of the anterior side of the specimen. 



Geological formation and locality. In the Lower Coal measures : Des- 

 moines valley, Iowa. 



Macrocheilus primogenms. 



PLATE XXIX. FIG. 11 a, b. 



Stylifer primogenia : CONRAD, Transactions of the Geological Society of Pennsylvania, 1835, 



Vol. i, pa. 267, pi. 12, f. 2. 



Fusus? inhabilis : MORTON in Silliman's Am. Journal of Science, Vol. xxix, 1836, pa. 160 



(appendix), pi. 3, f. 19 [also pi. 2, f. 14]. 

 Plectostylus, new genus : CONRAD, 1842. 



Macrocheilus inhabilis : NORWOOD & PRATTEN, Journal of the Academy of Nat. Sciences, 



Philadelphia, 1855, pa. 76, pi. 9, f. 9 a, b. 



Compare Macrocheilus ponderosus, SWALLOW, Transactions of the Academy of Sciences, 



St.Louis, Vol. i, no. 2, p. 7. 



SHELL ovoid, thick, ventricose. Spire short, tapering ab- 

 ruptly. Volutions rounded, five or more, the last one very 

 ventricose, and more than twice as long as the spire above. 

 Suture well defined, and sometimes a slight depression just 

 without the suture-line : aperture more than half the length 

 of the shell, oblong-ovate ; columella with a strong fold or 

 callosity ; outer lip thin and sharp. 



SURFACE marked by striaB parallel to the lines of growth, 

 which are sometimes more conspicuous on the upper part of 

 the volution. 



I can have little doubt but this is the shell originally described by Mr. CONRAD 

 in 1835, and in the succeeding year by Dr. MORTON, from casts in the collection of 

 Dr. HILDRETH. It has been recognized by Messrs. NORWOOD & PRATTEN as identical 

 with that described by Dr. MORTON, and the M. ponderosus of SWALLOW seems to 

 be nearly related to the shell under consideration. 



Fig. 11 a. View of the aperture and columella. 

 Fig. 11 b. Anterior side of the same shell. 



Geological position and localities. In the Coal measures of Ohio, Indiana, 

 Illinois and Iowa. 



