1856.] 69 



lateral slopes on each side of it. Six small indistinct strife radiate from the 

 apex, two of which are directed forwards and ontwards, and four backwards and 

 outwards; faint, irregular, concentric undulations dimly indicate the progressive 

 stages of growth ; surface otherwise smooth ; aperture circular. Diameter of 

 aperture 1.93 inches, heigh th of apex .80 inch. 

 Locality and position. Same as preceding. 



Dentalium fragilis. Shell very thin, slender, gently curved from npex to 

 base ; surface ornamented by rounded, thread-like, longitudinal lines, Avhich are 

 crossed somewhat obliquely upwards from the inner to the outer side of the curve 

 by ver}' fine regular lines of growth. The longitudinal lines terminate abruptly 

 a little below the apex, (where they number about fourteen) and increase in 

 number by implantation, and diminish in size and regularity towards the larger 

 end, so as to become nearly obsolete on the lower half of the shell ; aperture 

 circular. Diameter at the apex .05 inch, do. of a fragment .85 inch, below apex 

 .13 inch, do. of largest fragment .19 inch; thickness of shell .02 inch; apical 

 angle 3. 



The above species is closely allied to D. gracilis, Hall and Jleek, (vol. 5, new 

 series. Trans. Acad. Arts and Sciences, Boston) but differs in having much less 

 prominent longitudinal striie, and not more than half as thick a shell ; while 

 the aperture, which in the former is nearly always subcircular, in the present 

 species is exactly circular. 



Locality and position. Yellow Stone river, one hundred and fifty miles above 

 mouth. No. 4 of section. 



Bulla volvaria. Shell narrow ovate, tapering gradually from below the 

 middle upwards, and extended obliquely below ; spire hidden, sometimes ura- 

 bilicate ; surface ornamented by faint lines of growth and shallow revolving 

 striiB, about one fifth as wide as the spaces between on the middle of the shell, 

 but more closely crowded and irregular at the lower and upper extremities. 

 Occasionally a much finer stria occupies one of the spaces between the others ; 

 aperture long, narrow, gently curved, rising above the summit of the body, about 

 half as wide above the middle as below ; lip thin, closely folded over the rudi- 

 mentary umbilicus. Length .66 inch, breadth .36 inch. 



Loccility and position Moreau trading post. No. 5 of section. 



BcLLA MINOR. Shell minute, ovate, or elliptical ; spire hidden, umbilicate ; 

 surface marked by elevated, sharp, revolving strife, about equal to the grooves 

 between ; fine sharply elevated lines of growth, more distinct in the grooves than 

 on the elevations, mark the surface in the other direction; aperture large, rising 

 above the summit of the body, narrow and obtuse above, and widening gradu- 

 ally below. Length about .10 inch, breadth .05. 



Locality and position. Moreau trading post. No. 5 of the series. 



Bulla occidentalis. Shell oval or ovate, thin, ventricose, widest a little above the 

 middle, rounded at the summit and somewhat obliquely extended in front; spire 

 hidden, umbilicate; surface unknown. Indistinct lines of groAvth, and stronger 

 revolving striae less than the spaces between are seen on the cast ; aperture 

 curved, narrow behind, risingabove the summit of the bodj^, and wideningcliiefly 

 on the inner side below; lip slightly curved outwards round the lower part of 

 the aperture. Length .44 inch, breadth .28 inch. 



Locality and position. Yellow stone river. No. 4 of the series. 



"We have other new species of Gasteropoda from the Cretaceous formations of 

 the upper Missouri, but refrain from indicating them out of deference to friends 

 having specimens of the same which they wish to describe. 



