NATURAL SCIENCES OF PHILADELPHIA. 269 



spire scarcely visible above the body whorl in a side view ; volutions three 

 and a half to four, increasing moderately in breadth, last one sloping with a 

 moderate convexity between the suture and the periphery, and slightly ex- 

 cavated in the umbilical region ; suture not impressed ; aperture transversely 

 suboval, being rounded on the outer side and straightened on the lower half of 

 the inner side, but modified by the return of the body whorl above. Surface 

 showing scarcely any traces of lines of growth, even under a good magnifier. 

 (Type of the genus.) 



Breadth of a large specimen 019 inch ; height f>07 inch. 



Locality and position. Hodge's creek, Macoupen County, Illinois ; Lower 

 Coal Measures. 



Genus Mickodoma, M. & W. 



Shell small, rather thick, conical, imperforate, composed of flattened whorls, 

 the last one of which is more or less angular around the midille and little pro- 

 duced below ; aperture about as high as wide ; outer lip simple, straight, and 

 oblique in outline ; columella without folds or plications ; inner lip thin and 

 slightly reflexed at the base of the columella. Surface with revolving nodu- 

 lar ridges. 



We have for several years past had under consideration a number of good 

 specimens of the little shell, for the reception of which this genus is proposed, 

 but delayed publishing a description of it because we were iu doubt respect- 

 ing its generic relations. At afiist glance it presents much the appearance of 

 a Murchisonia, or a rather elongated Pleurotom tria ; but even where the outer 

 lip is broken away, so that the sinus characteristic of these genera could not 

 be seen if it had existed, an examination under a good lense shows 

 that it has no revolving band, and that its lines of growth are without the 

 peculiar curve in passing across the whorls, so invariably accompanying the 

 sinus in the lip of Murchisonia and other shells of that type. It also resembles 

 some of the small, short species of Turritella, but in addition to its shorter, 

 trochiform outline, its outer lip presents an obliquity and straightness of out- 

 line that imparts a peculiar appearance to the aperture, not seen in that genus. 

 From our genus Orthonema, with which it is associated in the rocks, it differs, 

 not only in its shorter trochiform outline and nodular revolving ridges, but 

 also in its very oblique lines of growth and the consequent obliquity of its 

 outer lip. 



It is not easy to determine the family affinities of this type, but it may pos- 

 sibly belong to the Littorinidce. It is probable that Pleuratomaria serrilimba 

 and P. biseriata, of Phillips, referred by Prof, de Koninck to the genus Troc/ius, 

 may belong to this genus. We doubt the existence of the genus Trockus, 

 as properly restricted to such types as the recent T. niloticus, Linnaeus, during 

 the Carboniferous epoch. 



MlCRODOMA CONICA, M. & W. 



Shell rather elongate conical or subtrochiform ; volutions seven, flattened 

 on a line with the slope of the spire, increasing rather gradually in size last 

 one not much produced below the mesial angle, where it is only marked by 

 minute striae of growth ; suture rather deep ; aperture quadrato-suborbicular. 

 Surface ornamented by three distinct, revolving, nodular ridges, the largest 

 and lowest of which occupies the mesial angle of the body whorl, and passes 

 around immediately above the suture of the whorls of the spire, while the 

 upper one occupies the tipper margin of all the whorls just below the suture, 

 and the third one passes around midway between the others. Lines of growth 

 small and crossing the flattened slopiug sides of each whorl obliquely, so as 

 to indicate a distinct forward extension of the outer lip at its connection with the 

 body whorl above. Nodes of the revolving angles small, closely and regularly 

 arranged on the different ridges, so as to form oblique rows parallel to the 

 lines of growth. 



Length, 0-21 inch ; breadth, 0-12 inch ; apical angle, 36. 



1866.] 



