598 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



are sometimes absent on the base. The lip-varix is bright green. On the 

 penultimate whorl the ground-color changes to purple-brown and the 

 markings disappear, the earlier whorls being uniform dark purple-brown. 

 The surface shows delicate growth-striae and a very minute, almost effaced 

 spiral striation. The spire is conic, with straight outlines, the apex entire 

 and obtuse, though small. Whorls 5, the early ones convex, the first 2% 

 being rounded, without trace of keels. A peripheral keel then begins, 

 strong from the beginning, and projecting flange-like above the suture. 

 The last whorl descends slowly from about its last third and much more 

 rapidly near the aperture. The peripheral keel projects very strongly and 

 is slightly undulating ; and a small keel arises below the suture, becomes 

 stronger on the back, then gradually decreases. On the base, midway 

 between the periphery and center, a low keel revolves, the area within it 

 being nearly flat. There is a very narrow crescentic columellar area. 

 The outer lip is strengthened by a very high and massive varix, which is 

 recurved above, with a rib on its face running to the lip-edge, and below 

 passes into the basal keel. The aperture is very oblique, ovate ; the outer 

 lip thin at the edge. The columella is narrowly calloused and regularly 

 concave. 



Length 5.5, diam. 6 mm. 

 4.6 " 4.7 " 



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

 Types, No. 60,689, A. N. S. P. 



Development. The neanic stage is sharply divided into 



(1) A lapidum substage, in which the shell is rounded, without keels, 

 comprising the first 2^ whorls. 



(2) A carinate substage, initiated by the almost abrupt rise of the periph- 

 eral carina. From a half whorl to a whorl this is the only keel developed. 

 This stage corresponds to the adult P. hidalgoi, and is of brief duration. 



(3) The basal and the subsutural carinae begin, weak at first, becoming 

 stronger near the end of the substage. 



The ephebic stage is announced by the expansion to form the varix, 

 which, however, is not terminal, the whorl continuing and contracting 

 beyond it, thus assuming gerontic characteristics. 



P. microthauma is related to P. hidalgoi and P. peristomatus, but it is 

 a much more evolved form than either, structures added in the second 



