INVERTEBRATE PALEONTOLOGY. 559 



Cerilhidea (Pircnella)! Ncbrasccnsis, M. &H. 



Plate 43, figs. 9, a, b, c (bis)." 



Cerithivm Nebraseense, Meek and Hayden (1850), Proceed. Acad. Nat. Sri. Philad., VIII, 125. 

 Cerithium (Cerithideal) Sebrascense, Meek and Hayden (1860), ib., XII, 430. 



Cerilhidea (Pirenella) Nebrascensin, Meek (1866), in Conrad's Smithsonian Check-List N. Am. Eocene 

 Invertebrate Fossils, 12. 



Shell elongate-conical, or subterete ; volutions apparently eight and a 

 half to nine when the spire is complete, convex, and ornamented by three 

 revolving rows of small, sharply-elevated, regularly-disposed granules; last 

 or body-whorl rounded below ; suture distinctly defined; surface marked by 

 fine, rather obscure lines of growth, crossed by more distinct, thread-like, 

 revolving lines, about three or four of which may usually he counted between 

 each two of the rows of granules; aperture a little oval, or nearly circular, 

 but terminating, on the inner side below, in a small, rather oblique sinus, or 

 short canal. 



Length of a specimen consisting of six of the lower whorls, 0.49 inch ; 

 breadth, 0.18 inch ; length of aperture, including the short canal, 0.14 inch; 

 breadth, 0.10 inch; apical angle regular, divergence 19°. 



On the lower part of the body-whorl, several of the revolving lines are 

 much larger than on other parts of the shell, and the upper two are some- 

 times studded with granules, so as to make on this whorl four or five, instead 

 of three rows. Occasionally, one or more of the subordinate intermediate 

 revolving lines on the lower part of the body-whorl also become minutely 

 granular. All the specimens yet seen have a few of the upper whorls, 

 and most of the lip broken away, and some of the succeeding turns eroded, 

 apparently as if the shell had been decollated during the life of the animal. 

 Until better specimens can be examined, the generic characters of this shell 

 cannot be determined beyond doubt. 



It should be stated here, for the information of Conchologists, who are 

 accustomed to examining perfect specimens of shells, that our figures do not 

 show the aperture and lip entire, but only as seen in broken specimens. So 

 far as known, its associates are fresh- and brackish-water types ; otherwise, I 

 should have been inclined to refer it to Cerithium, from all of its known 

 characters. 



Locality and position. — Head of Little Missouri, in apparently the lower 

 part of the Fort Union Lignite group (probably Eocene). 



* By an oversight, in adding some figures to this plate after it was made op, both this species and 

 a bivalve on the same plate were each numbered 9, a, b, c. 



