INVERTEBRATE PALEONTOLOGY. 275 



Owing to the difficulty of distinguishing this genus from Bulla and 

 several of the allied groups, as the shells are found in a fossilized condi- 

 tion, it is very difficult to arrive at satisfactory conclusions in regard to its 

 geological range. Forms closely resembling it occur in the Triassic and 

 Jurassic rocks. It seems to he represented by well-defined, and character- 

 istic species in the Cretaceous, and ranges through the Tertiary. In the 

 existing seas, it is also represented by a number of living species, including 

 those of both sections. They occur mainly in deep water, on the coasts of 

 the United States, Northern Europe, the Philippine Islands, Australia, &c. 



Cylichnal volvaria, M. &H. 



Plate 31, figs. 2, a, b." 



Unlln volvaria, Meek and Hayden (1856), Proceed. Acad. Nat Sci. Philad., VIII, 69.— Meek (1864), Smith- 

 sonian Cheek-List N. Am. Cret. Fossils, 16. 



Shell thin, elongate-subovate, being widest below the middle, and taper- 

 ing upward gradually, with moderately convex sides, to the summit, which 

 is narrowly rounded, with apparently a very small central pit; lower 

 extremity oblique and subangular, or narrowly rounded; aperture very nar- 

 row, gently arcuate, and not much expanded below, rising a little above the 

 summit of the body at the upper extremity; inner lip thin, reflexed, and 

 closely appressed below, where it shows a faint oblique fold or ridge ; sur- 

 face marked by fine, obscure lines of growth, which are crossed by shallow, 

 transverse strife, narrower than the spaces between, excepting on the upper 

 and lower extremities, where they are more crowded, and more distinctly 

 defined. 



Length, 0.68 inch ; breadth, 0.37 inch ; breadth of aperture at its widest 

 part below, 0.18 inch ; breadth of same near upper extremity, 0.07 inch. 



This species will be readily distinguished from all of the other shells of 

 this type yet found in the Upper Missouri rocks, by its narrower, propor- 

 tionally longer form, tapering from below the middle to its narrower upper 

 extremity ; and by its narrower aperture. In these characters, it resembles our 

 Haminea minor, figured on the same plate, more nearly than any of the others, 

 but is still proportionally more elongated, and has a differently-formed aper- 

 ture, which is less arcuate and not so wide below. In size and general 

 appearance, it is more nearly like Cylichna costata, Gabb, from the Creta- 

 ceous rocks of California ; but, as may be seen by the figure, it is more regu- 



" uur figured specimen, having tin- lip broken, is represented too angular at the base; and fig. 

 •J. h. is defective in not having the lip projecting a little above. 



