572 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



upon the Naticoid form much earher in some species than in others ; and 

 judging- from these various degrees of acceleration, it would seem that the 

 specialized species are of unequal antiquity. 



Key to Species and Subspecies of Potamolithus. 



I. Columella wide, having a longitudinal furrow excavated in its face. Latter part of the suture 



not more rapidly descending ; outer lip strengthened by a strong varix. 



a. Last whorl dominated by a very strong peripheral keel. P. riisliii. 



a}. Last whorl flattened peripherally, without carina. P. pliilippiamis. 



c^. Last whorl rounded, conspicuously banded with green. P. ilieringi. 



n. No longitudinal groove in the face of the columella. 



a. Outer lip well expanded or with a prominent varix ; not notched or sinuous. 



b. Periphery strongly keeled, the keel visible on part of the penultimate whorl. 



c. Three keels on the last whorl. P. viicrothauma. 



f'. Last whorl flattened above and below the peripheral keel. P. hidalgoi. 



c^. Last whorl globose, convex above and below the peripheral keel. 



P. peristomatus. 

 b^. Periphery obtuse or rounded. 



c. Lip-varix very strong ; periphery hardly angular, base convex ; back with a 



spiral rib below the suture ; no columellar area. P. dinochihts. 



c'. Lip-varix narrow ; last whorl without keels, e.xcept that around the columellar 



area. /. orbignyi. 



rt'. Outer lip simple or slightly contracted, without an e-xternal varix, the edge not sinuous. 



b. Last whorl sculptured with spiral keels or angles. 



c. With a single keel or angle at the basal periphery, none above it. 



d. High-trochiform, flattened above and below the strong carina, much higher 



than wide ; columella narrow. P. conicus. 



d^. Obliquely trochiform, convex above the peripheral angle, about as wide as 



high ; columella wide. P. bnschii. 



c^. Trochiform, with a carina at the basal periphery and two contiguous keels on 



the back above. P. tricostatus. 



c^. Trochiform, with a strong carina at the median peripheiy, the slope above it 



flat, with a small carina at the upper third ; no distinct umbilical area. 



P. carinifcr. 



c^. .Stout keels at both periphery and shoulder, giving the last whorl a squarish 



contour ; umbilical area ample ; spire very short. P. quadratns. 



c*. Base and periphery rounded, a shallow sulcus or two low ridges on the back 



above. P. lapidum superstdcatiis. 



r\ Back of last whorl with two contiguous angles, the upper one stronger, base 



rounded, spire rather slender and high. P. hatcheri. 



b^. Last whorl rounded, without keels, angles or sulci. 



c. Shape approaching globular, the spire short or very short, conic. 

 d. Columella narrow ; last whorl evenly rounded. 



c. Green or olivaceous ; alt. 5, diam. 4 mm. P. lapidum. 



e\ Yellow, alt. 3.3, diam. 3 mm. P. para^iensis. 



