858 THE PALEONTOLOGY OF MINNESOTA. 



[Cyrtolites. 



whorls but becoming less angular if not quite obsolete in nearing the apertural 

 margin; slit-band occasionally distinguishable; outer lip of aperture thin, sinuate 

 and notched centrally; inner lip entire where it touches the preceding volution; 

 within the edge it is first deeply concave, then produced into a broad thick flattened 

 plate or septum, which extends a considerable distance into the aperture; upper 

 surface of septum with a low but well defined median ridge; inner aperture covered 

 (always?) by a triangular flat operculum. Type, C. carinata Hall. 



C. acuta U. &S. Black River group. C. carinata Hall. Trenton group. 



C. minima U. & 8. Black River group. C. cunulce Hall. Trenton group. 



C. phakra Sardeson. Black River group. C. explanata Dlrich. Top of Trenton group. 



C. cymbula Hall. "Hudson River group." 



Genus CYRTOLITES, Conrad. 



Cyrtolites, CONRAD, 1838, Ann. Rept. Nat. Hist. Surv. N. Y., p. 118. 



Cyrtolites (part.), Hall, 1847, Pal. N. Y., vol. i, p. 187. MILLBB, 1874, Cin. Quart. Jour. Sci., vol. i, p. 

 308; also 1889, N. A. Geol. and Pal., p. 401. WAAGEN, 1880, Pal. Indica, 

 ser. 13, pt. 2, p. 132. LINDSTROM, 1884, Silurian Gastropoda of Gotland, 

 p. 81. 



For generic characters see page 846. 



Restricting this genus to species of the type of C. ornatus Conrad, we have 

 an isolated group of a shells that we find most difficult to classify satisfactorily. 

 Despite the fact that authors have so generally agreed in uniting with the group 

 that other peculiar type which we separate as Conradella, there is in reality but 

 very little reason for considering them as related and much less as identical. ' In 

 discussing that genus on a following page it is shown, we believe, to the satisfac- 

 tion of every fair-minded student that Conradella, with its long dorsal slit and 

 imbricated lamellae, is nearer Bucania, Salpingostoma and Tremanotus than to 

 Cyrtolites. Indeed, we cannot see how an unprejudiced comparison of Conradella 

 and Cyrtolites can fail to impress the observer with the conviction that the two 

 groups of species are not only generically distinct, but not even closely related. 



Cyrtolites has been loosely employed by most authors for symmetrically in- 

 volute, disciform shells, having the dorsum carinate or angular and the umbilicus 

 broad so as to expose the inner volutions. These characters pertain to several 

 widely distinct genera, and to use them as characteristic of a single genus is to bring 

 together a most heterogeneous assemblage of forms. Thus we have among the 

 species that have been referred to Cyrtolites several belonging to Oxydiscus, Koken 

 (Tropidiscus, Meek), a genus that is nearer Bucania and Bellerophon ; several of 

 Conrndella, which, as we have said, is nearer Bucania; and all of the species com- 

 prisod in the new genus Cyrtolilina. Lindstrom and others have thought that 

 PorceUia, even, is the same as Cyrtolites. With equal propriety we might refer to 

 the same generic group also the recent genus Atlanta! 



