496 Systematic Paleontology 



The only evidence of the former existence of this species within the area 

 under discussion is a single cast from along the Chesapeake and Delaware 

 Canal. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey, U. S. National Museum. 



Outside Distribution. — Monmouth Formation. Navesink marl, New- 

 Jersey. Selma Chalk. Wilcox County, Alabama; east-central Missis- 

 sippi. 



Family NATICIDAE 



Genus GYRODES Conrad 

 [Jour. Acad. Nat. Sci., Phila., 2d ser., vol. iv, 1860, p. 289] 



Type. — Natica (Gyrodes) crenata Conrad. 



Shell very thin, low, sub globose, whorls flattened posteriorly. Sutural 

 channel developed in a greater or less degree, the outer margin of the 

 channel usually wrinkled or crenulate. External sculpture not present 

 upon the periphery of the whorl. Aperture obliquely ovate. Both 

 labrum and labram thin ; umbilicus usually wide, uniformly deep and free 

 from callus. 



The diagnostic characters of Gyrodes are the thin shell, the depressed 

 globose whorls, the frequently crenate sutural channel margin and the 

 deep and open umbilicus. 



A. Whorls oblique, asymmetrically rounded, outer margin of sutural 



channel acutely angulated Gyrodes petrosus 



B. Whorls erect, symmetrically rounded, outer margin of sutural chan- 



nel acute Gyrodes abyssinus 



Gyrodes petrosus (Morton) Gabb 

 Plate XIII, Fig. 8 



Natica petrosa Morton, 1834, Syn. Org. Rem. Cret. Group, U. S., p. 48, pi. 



xix, fig. 6. 

 Natica alveata Conrad, 1860, Jour. Acad. Nat. Sci., Phila., 2d ser., vol. iv, 



p. 289, pi. xlvi, fig. 45. 

 Gyrodes alveata Meek, 1864, Check List Inv. Fossils, N. A., Cret. and Jur., 



p. 21. 

 Gyrodes petrosa Meek, 1864, Ibidem. 



Gyrodes petrosus Conrad, 1868, Cook's Geol. of New Jersey, p. 729. 

 Gyrodes petrosa Gabb, 1876, Proc. Acad. Nat. Sci., Phila., p. 295. 

 Gyrodes petrosus Whitfield, 1892, Mon. TJ. S. Geol. Survey, vol. xviii, p. 127, 



pi. xvi, figs. 1-4. 



Etymology: yipos, circle; oi'Sos, like. 



