600 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



of the upper surface is also of late appearance, these structures belonging 

 to the third neanic substage, the second, or unicarinate, substage thus 

 occupying the greater part of the neanic stage. The discontinuation of 

 the upper ridge or hump initiates the ephebic substage. The marginal 

 varix and the absence of any tendency of the last whorl to descend or 

 loosen its coil anteriorly, show that this species is at its acme. It has 

 none of the stigmata of senility which are so manifest in P. micro- 

 thauma, P. hidalgoi, etc. 



There is some variation in the degree of depression of the whole shell, 

 the amplitude of the columellar area and in the prominence of the hump 

 on the back, which is sometimes almost suppressed. The size also varies, 

 one specimen before me with the varix nearly complete measuring only 

 5 mm. in diameter. 



The relationship between P. rushii and P. iheringi is exceedingly 

 interesting. The two species are similar in general color-scheme, in the 

 varix, absence of more rapid descent of the suture towards the mouth, 

 etc., but are totally diverse in contour, the one being carinate, the other 

 smooth and Naticoid. Yet it is significant that while P. iheringi has no 

 trace of a peripheral keel, the green band occupies the same position as 

 that coloring the keel in P. rushii. 



POTAMOLITHUS IHERINGI Pilsbiy. 

 (Plate XXXIX, Figs. 2, za.) 



Potamolithus iheringi Pilsbry, Nautilus, X, p. 87, Dec., 1896. 



The imperforate or rimate shell is globular-conic, very solid and strong, 

 pale olive-yellow, with a sharply defined, bright green band bordering the 

 suture below and another immediately above the periphery, visible as a 

 narrow border on the penultimate whorl above the suture ; the columellar 

 area being also dull green. Very faint growth-lines and an almost effaced 

 spiral striation are visible under the lens. The spire is low conoidal, the 

 suture clearly incised but not impressed. Whorls 4^ , the first half whorl 

 usually lost in adult shells ; the rest are slightly convex, the last whorl 

 being globular, at first regularly rounded, but its last third descends slowly, 

 and is perceptibly flattened obliquely in the peripheral region, being more 

 full and convex below. The base is convex, but there is a concave cres- 

 centic columellar area defined by an inconspicuous angle. The outer lip 

 is strengthened by a moderately strong varix, bevelled to the lip-edge, 



