57 2 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



upon the Naticoid form much earlier 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. rushii. 



a 1 . Last whorl flattened peripherally, without carina. P. philippianus. 



a 2 . Last whorl rounded, conspicuously banded with green. P. iheringi. 



II. 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. microthauma. 



c 1 . Last whorl flattened above and below the peripheral keel. P. hidalgoi. 



<*. Last whorl globose, convex above and below the peripheral keel. 



P. peristomatus. 

 b 1 . 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 1 . Lip-varix narrow ; last whorl without keels, except that around the columellar 



area. /. orbignyi. 



a 1 . Outer lip simple or slightly contracted, without an external 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 l . Obliquely trochiform, convex above the peripheral angle, about as wide as 



high ; columella wide. P. buschii. 



c 1 . 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 periphery, the slope above it 



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



P. carinifer. 



c 3 . Stout keels at both periphery and shoulder, giving the last whorl a squarish 



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



c 4 . Base and periphery rounded, a shallow sulcus or two low ridges on the back 



above. P. lapidnm supersulcatus. 



c 6 . Back of last whorl with two contiguous angles, the upper one stronger, base 



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



b 1 . 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. 



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



e l . Yellow, alt. 3.3, diam. 3 mm. P. paranensis. 





