INVERTEBRATE PAL/EONTOLOGY. 309 



angular instead of rounded. Dr. Stoliczka's figures of the Indian shell show 

 the impressions, on the cast of the interior, of the internal varices, exactly as 

 in our species. If the latter, however, has no plaits on the columella, their 

 presence on that of the Indian shell would, of course, be a very strongly- 

 marked distinction. This point, however, remains to be settled when other 

 collections can he examined ; though I have seen the columella of our speci- 

 mens far enough around within the aperture to lead at least to the impres- 

 sion that it is most probably destitute of plaits. 



Local i tij and position. — -The original typical specimens of this shell were 

 found on Sage Creek, Dakota, where they occur in the upper beds of the 

 Fort Pierre group of the Upper Missouri Cretaceous series. Our figured 

 specimen also came from the same position on Cheyenne River, of which 

 Sage Creek is a tributary. 



NATICID^E. 



Genus GYRODES, Conrad. 



Synon. — Xatica (sp.), Micheliu, d'Orbigny, Pictet and Roux, and others. 



Gyrodcs, Conrad (1660), Jour. Acad. Nat. Soi. Philad., IV, 289 (aa a subgenus under Xatica). — 

 Gabb (1801), Proceed. Acad. Nat. Sci. Philad., XIII, 32; and (1804) Palajont. Califor- 

 nia, I, 108 (as a genus). — Meek (1804), Smithsonian Check-List N. Am. Cret. Fossils, 

 21 (as a genus). — Stoliczka (1808), Palasont. Indica, II, 297 (as a genus). 



Elym. — yvpoeidfc, like a circle; in allusion to the revolving umbilical margin. 

 Examp. — Natica (Gyrodes) crenata, Con. 



Shell more or less depressed-subglobose ; aperture rhombic or subovate, 

 and generally angular or narrowly rounded below; inner lip thin; umbilicus 

 wide, deep, and without any traces of a callosity, bounded by a revolving, 

 more or less crenate, carina, and sometimes provided with a second small 

 revolving ridge within ; volutions truncated above, so as to give the suture a 

 canaliculate appearance, the outer edge of the truncation being generally 

 wrinkled or crenate ; general surface showing merely lines of growth, with 

 sometimes faint traces of microscopic revolving striae. 



This genus is readily distinguished by its thin shell, wide, open umbilicus, 

 bounded by an angular, more or less crenate, margin, and without a trace of a 

 callosity within, as well as by the truncated, slightly concave, and more or 

 less wrinkled upper edge of its volutions — a combination of characters 

 unknown in any other type of the Naticidce. The species have generally 

 been referred, to the genus Natica by palaeontologists, who are too often far 

 behind the present state of conchological science. Few well-informed eon- 



