582 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



Uruguay River at Paysandu, Uruguay, under stones at extreme low 



water. 



Development. The youngest shells at hand lack about one whorl of 

 completion. At this stage the shape is practically that of adults, except 

 that there is no columellar crescent and the columella is wide and flat. 

 The weak subsutural ridge, when developed at all, appears on the back 

 of the last whorl only. The most fully developed shells therefore have 

 the characters of the four-whorled stage in P. tricostatus, while most 

 shells, wanting the upper keel, are like tricostatus at the three-whorled 

 stage. 



P. conicus differs from P. tricostattts by its smaller, size, more highly 

 conic shape, and the less developed sculpture of the last whorl. It is 

 more closely related to P. buschii, both having the columella wide during 

 the neanic stage ; but in the adult stage P. btischii is depressed and P. 

 conicus elevated in shape. 



The color in life is probably always more or less green. The rich brown 

 tint of those figured may be due to change in alcohol, though I am not 

 sure that this is the case, as the shells were dry when they came into my 

 possession. A lot of P. buschii which had been in alcohol have changed 

 to brown, while all of those dried fresh are green. 



POTAMOLITHUS ORBIGNYI Pilsbry. 

 (Plate XL, Figs. 1-5.) 



Potamolithus orbignyi Pilsbry, Nautilus, X, p. 88, Dec., 1896. 



The shell is imperforate, globose, solid and strong. Uniform olive 

 colored, or with brown bands below the suture and in the middle of the 

 last whorl, on an olive or green ground. Surface smoothish, with faint 

 growth-lines and fine, very indistinct spiral striae. The spire is very short 

 and conic. Whorls nearly 4^ , strongly convex, the later third of the last 

 whorl descending more rapidly. The last whorl is squarish, obtusely 

 biangular, being shouldered above, flattened in the middle, and more or 

 less angular at the basal periphery. The ample crescentic columellar area 

 is concave and bounded by an angle. The lip is strengthened by a narrow 

 varix, which is blackish and bevelled to the lip-edge. The aperture is 

 very oblique, and rounded-ovate. The outer lip is thickened within, the 

 inner lip heavily calloused. The columella is broad and flattened. 



