594 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



upper and lower keels are already strongly developed near the lip, but 

 there is no trace of the intermediate keel, which in adults is less than one 

 whorl long. The two keels present are very weak at the beginning of 

 the fourth whorl, the basal one stronger there than the upper, so that 

 apparently at 2^ whorls the form must be Naticoid, like P. lapidum. 

 The next stage, at about 3 whorls, would have a basal angle only, like 

 P. bitsc/iii. The columella is narrow, as in P. lapidum, throughout the 

 neanic stage. The ephebic stage is marked by a slight expansion, forming 

 a narrow and low varix. Fully adult shells are markedly gerontic by 

 reason of the strong post-variceal contraction. 



The figures of this species given by Brot are unsatisfactory from being 

 too small properly to show its characters. The median keel is shown too 

 low in position. The original locality was indefinite, but comprised an 

 area including the river in the neighborhood of Paysandu. Three speci- 

 mens of the original lot, received from Dr. Brot, agree with those col- 

 lected by Dr. Rush at Paysandu. 



POTAMOLITHUS HATCHERI Sp. nOV. 

 (Plate XXXVIII, Figs. 6, 6a.) 



The shell is imperforate, turbinate, solid and strong, pale yellowish- 

 green, becoming darker and narrowly streaked with dark green on the later 

 half of the last whorl ; on the penultimate whorl the color changes to rich 

 reddish-brown, becoming darker towards the summit. Surface glossy, with 

 faint growth-lines and fine, indistinct spiral striae. Spire conic, high and 

 rather slender, the apex lost in the type, a pit in its place. Three whorls 

 remain, the first two of them evenly convex. The last fourth of the 

 penultimate whorl is very obscurely biangular, the lower angle more dis- 

 tinct than the other. These angles are more pronounced on the face of 

 the last whorl, and on its later half the upper angle rises into a strong keel, 

 the surface above it concave ; at the same time, the lower angle loses in 

 prominence, and is quite lost behind the lip. An extremely weak spiral 

 ridge is developed on the back below the suture. The base is rounded. 

 The last third of a volution of the suture descends more rapidly. There 

 is no trace of a varix at the lip. The aperture is quite oblique, as wide as 

 high, almost circular, but slightly angular above. The lip is obtuse, thick- 

 ened within, with a continuous blackish marginal line. The columella is 



