592 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



POTAMOLITHUS QUADRATUS Pilsbry & von Ihering, sp. nov. 



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



The shell is solid, imperforate, light olivaceous-brown, lightly marked 

 with growth-lines, shining. The spire is very short, y/ 2 whorls remain- 

 ing, the apex being eroded. The penultimate whorl has a strong angle 

 at its upper third, is flat and horizontal above the angle, flat and sloping 

 below it. The last whorl is bicarinate, the peripheral and shoulder carinae 

 being about equally strong, giving it a square appearance. The base 

 tapers to a rather high ridge, which bounds a wide, excavated, umbilical 

 area. The peripheral angle becomes obsolete just behind the outer lip, 

 and it is visible only on the last whorl. The aperture is very oblique, 

 ovate, white within. Peristome a little contracted, its face flat and thick- 

 ened within, at and above the periphery and in the upper angle ; below 

 the periphery it is thin and a little retracted. The columella and parietal 

 wall are moderately calloused, a groove in the face running parallel to 

 their outer margin. 



Length 3.9, diam. 4 mm. 



Paysandu, Uruguay. Dr. Fl. Filippone. 



This snail differs from P. carinifer in the shape of the last whorl, the 

 much stronger upper keel (which is developed earlier) and in having a 

 large umbilical area. It lacks the median keel and variegated colora- 

 tion of P. tricostatus. 



A young shell 2.4 mm. diam. consists of 2^ whorls, the first 15^ are 

 rounded ; the keel at the shoulder then begins, rapidly becoming 

 strong. The columella is very wide, its face excavated. 



POTAMOLITHUS CARINIFER sp. nov. 



(Plate XLItf, Figs. 5, 50.) 



The shell is imperforate, trochiform, solid, blackish-brown in old 

 individuals, the shell of a brick-red tint below the cuticle. Surface lightly 

 marked with growth-lines. The spire is very short, conoidal. Whorls 

 about 4, the earlier ones convex. The first half of the penultimate whorl 

 is convex ; then a small carina gradually arises a short distance below 

 the suture, the surface flat and sloping below it ; a little later a strongly 

 projecting keel appears just above the lower suture, being uncovered by 

 the descent of the last whorl. The last whorl has a strongly projecting 



