564 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



It is possible that these shells may be found, owing to the prominence 

 of their outer lip below, and its sinuous character above (not well represented 

 in any of our figures on plate 42), to fall into the group Trypanostoma 

 (= Pleurocera), and thus have to be called Pleurocera or Trypanostoma con- 

 vexa. The name Pleurocera, Raf, has priority over Trypanostoma, Lea; but 

 it would seem to me that the two sections might be both retained in a sub- 

 generic sense with convenience, though Mr. Tryon, who has given much 

 attention to the study of these and allied groups, thinks them undistinguish- 

 able, owing to the gradual blending of the two types through intermediate 

 species. 



Locality and position. — Same as last, at the mouth of Judith River, 



Montana. 



Goniobasis invenusta, M. &H. 



Plate 42, figs. 1, a, 6, c, d, e. 



Melania invenusta, Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Pbilad., IX, 137. 



Shell thin, conical-subovate, spire moderately elevated, not eroded; 

 volutions about seven to seven and a half, very slightly convex, increasing 

 rather gradually from the apex ; suture nearly liuear, and not very strongly 

 impressed; surface generally almost smooth on the upper whorls, but having 

 fine obscure lines of growth, which become more distinct on the lower volu- 

 tions, where they are somewhat sigmoid, and crossed by nearly obsolete 

 revolving striae, usually most distinct on the upper part of the last whorl ; 

 aperture subovate, more or less oblique, narrowly rounded, but not distinctly 

 angular below. 



Length, about 0.90 inch ; breadth, nearly 0.40 inch ; apical angle 

 regular, divergence variable, but most generally about 27°. 



It is rather difficult to determine from our imperfect specimens whether 

 or not this species presents the characters of the genus Goniobasis. Unfor- 

 tunately none of them are in a condition to convey a \ery clear idea ot the 

 nature of the aperture; but, as nearly as can be determined from the lines of 

 growth, the outline of the lip is not quite so strongly sinuous as in Trypano- 

 stoma. The base of its aperture, however, is not so angular as in the more 

 typical species of that genus, though it differs from that of true Melania. 



Specifically, this shell is similar in some respects to immature specimens 

 of our Goniobasis convexa It differs, however, from the adult form ot that 

 species in having one or two whorls less, while its surface is smoother and 



