1889.] NATURAL SCIENCES OF PHILADELPHIA. 305 



have unquestionably been erroneously assigned to the genus. The 

 typical examples are for the most part from the Devonic and 

 Carbonic, and, although the group probably continued to flourish 

 after the close of the Paleozoic, it is very doubtful whether the 

 majority of the later forms can properly be regarded as congeneric. 

 In Europe the group became greatly expanded during the later 

 Devonic and Carbonic, but in America it is almost wholly confined to 

 the latter age, the other forms referred to the genus being, with 

 perhaps two or three exceptions, referable to other groups. 



Polyphemopsis of Portlock has commonly been considered synony- 

 mous with Macrochilus, but whether it can be regarded as identical 

 with the group as now defined cannot, at present, be satisfactorily 

 determined. Portlock's genus was founded upon such imperfect 

 material as to hardly deserve recognition in any case, and it would 

 probably simplify matters greatly to ignore the term altogether. 

 There appear to be no good grounds for assigning any American 

 gasteropods to Polyphemopsis. The species so referred have, in real- 

 ity, other generic affinities. 



The shells of the second section are subglobose, with the spire 

 relatively very small, short; the whorls convex, very rapidly expand- 

 ing, the last ventricose, and forming, by far, the greater part of the 

 shell; aperture oval; columella thickened, sometimes exhibiting 

 obsolete traces of an obtuse angularity; test comparatively much 

 thinner than in Soleniscus. Typified by Macrochilus ponderosum 

 Swallow and M. texanum Shumard. 



The enormous size of the body-whorl compared with the spire, and 

 the undeveloped columellar fold readily distinguish this form from 

 that of Soleniscus. In America the genus ranges from the Upper 

 Helderberg to the close of the Carbonic. From the evidence at 

 hand it appears that the species of this group were more strictly 

 marine in their habitat than the members of the first genus. 



For this group, as here defined, the term Sphcerodoma is proposed. 

 It is thought to embrace the following forms : — 



Sphaerodoma cooperensis (Swallow). 



Macrochilus cooperen.se Swallow, 1803. Trans. St. Louis Acad. 

 Sci., vol. II, p. 100. Kaskaskia limestone. 



Sphaerodoma littonana (Hall). 



Natica littonana Hall, 1858. Trans. Albany Inst., vol. IV, p. 30; 

 Naticopsis littonana Meek and Worthen, 18(i6. Proc. Acad. Nat. 

 Sci. Phila., p. 268; Macrochilus littonanum Whitfield, 1882. Bui. 

 Am. Mas. Nat. Hist., vol. I, p. 73. Warsaw limestone. 

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