INVERTEBRATE PALAEONTOLOGY. 271 



of Leach. In Atys, however, the shells are not only more solid, as above stated, 

 hut generally have the transverse st nation confined to the upper and lower 

 portions of the shell, instead of covering the whole surface; while, in most 

 cases, the form of Atys is different, and its columella ends below in a fold or 

 tooth, and the outer lip is twisted and produced above. 



II a in i ii c a in ■ i i <l « - 1 1 1 : 1 1 1 s , M. & H. 



Plate 18, figs. 11, a, b, and 12, a, b. 



Bulla occidentalis, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 69 (not B. occidentalis, 



A. Adams, of earlier date). 

 Bulla Xeirascensis, Meek and Hayden (1801), Proceed. Acad. Nat. Sci. Philad., XIII, 427.— Meek (1864), 



Smithsonian Check-List N. Am. Cret. Fossils, 16. 



Shell exceedingly thin, subovate, being generally a little widest slightly 

 below the middle; upper end a little obliquely subtruncated, and rounding 

 into the small umbilicoid central pit or perforation marking the position 

 of the sunken spire ; aperture narrow, arcuate, widening moderately, and 

 rather narrowly rounded below ; inner lip very thin, and rerlexed below, but 

 not always so closely appressed as quite to cover the very small, oblique 

 umbilical chink, very thinly spread on the body-volution above ; outer lip in 

 entire specimens rising somewhat above the summit, and rounding inward 

 posteriorly ; entire surface ornamented by very small, transverse, impressed 

 striae, separated by wider space, in each of which may sometimes be seen a 

 smaller one ; revolving striae usually more or less visible on internal casts. 



Length of a medium-sized specimen, 0.46 inch; breadth, 0.31 inch. 



This species resembles the recent Haminea solitaria of Say, from om 

 eastern coast, but is a little more ventricose, has stronger transverse striae 

 and its inner lip is a little wider and appressed below. The latter characters, 

 however, are not well represented in our figures, which are from internal 

 casts.* It is a very thin shell, being, not much thicker than strong, heavy 

 writing-paper; which fact seems to forbid its reference to Bulla proper. In 

 first describing it, we called it Bulla occidentalis ; but, on subsequently learn- 

 ing that A. Adams had previously described a recent Bulla under that name, 

 we proposed to call it B. Nebrascensis. In now referring it to the genus 

 Haminea, however, the original specific name has to be retained. 



Locality and position. — Yellowstone River, 150 miles above its mouth ; in 



* The magnified fig. 12, b, being from an internal cast, with the outer and lower margins broken 

 away, gives the erroneous appearance of the columella being strongly truncated below, which is not 

 natural. 



