562 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Goniobasis convexa, M. &H. 



Plate 42, figs. 2, a, b. 



f Turritella convexa, Meek and Haydeu (185(3), Proceed. Acad. Nat. Sci. Philad., VIII, 70. 

 Melania convexa, Meek and Hayden (1856), i6., 125. 



Fi e- 71 - Shell rather large, much elongated, terete ; volu- 



tions about nine or ten, flat, compactly wound, and in- 

 creasing gradually in size from the apex, last one 

 rounded below and not enlarged beyond the regular 

 increase of the others; suture linear ; surface marked 

 by rather fine lines of growth, crossed by regular equi- 

 distant, often obsolescent, thread-like revolving lines, 

 with sometimes between the latter traces of minute 

 Goniobasis convexa (type), striee ; aperture apparently rhombic-ovate; lip sharp, 



To show tbe curves of i 11 i i , i i 



,, ,. , ., broadly sinuous above, and prominent below. 



the lines of growth more » t 



correctly than figures 2, Length, about 1.60 inches ; breadth, 0.47 inch ; 



a, b, of plate 42. , ... 



length ot aperture, 0.40 inch ; apical angle convex, 

 divergence near the summit of the spire 30°, but not more than 20° below. 



The best specimen of this interesting species that I have seen, consists 

 of seven volutions, and appears to have lost two or three others from the apex, 

 while its aperture is a little distorted. The larger revolving lines (about 

 seven to ten of which may be counted on the second volution) are epiite 

 distinct, and nearly one-third as wide as the spaces between ; but the fine 

 revolving strise are closely crowded, and so small and obscure as to be only 

 seen by the aid of a good lens on perfectly-preserved specimens. On the 

 upper turns of the spire, the surface-sculpturing appears to differ somewhat 

 from that of the lower, the revolving lines being very faintly marked, and 

 the lines of growth between them sometimes assume the appearance of 

 microscopic granules. These smaller markings, however, are very minute 

 and obscure, and consequently will only be seen when carefully looked for, 

 on well-preserved specimens, by the aid of a magnifier. 



This shell has the general appearance of some of the elongated species 

 of Goniobasis ; but the curves of its lines of growth show that the outer lip 

 must have presented the prominence below, characteristic of the Trypano- 

 stuma (or Pleurocera) group. This curvature of the lines of growth, however, 

 is not well shown in our figures on plate 42, but is correctly represented in 

 the foregoing cut, figure 71. 



