The Genus Gyrotoma 25 



dwarfed. Larger Gyrotomae and larger Anculosae have been collected 

 in the same place. The student is led to wonder whether these dwarfed 

 shells do not dwell in some particular spot on these shoals where the con- 

 ditions of food supply, current, temperature or some other cause tend to 

 stunt molluscan growth as sometimes in very cold springs shell life is at 

 once small and plainly of mature development. 



Gyrotoma walkeri H. H. Smith, new species 

 PI. II, figs. 30, 31, and 32 



Shell: Cylindrical, shouldered, solid; body whorl slightly constricted, apex erod- 

 ed, base rounded and with only a suggestion of a sinus. Growth lines very fine, crossed 

 by delicate, waving lines to be seen on all parts of the exterior except where it is worn 

 smooth. Rest period scars rather rough, forming a few irregularly spaced nodules. 

 Fissure very shallow, very oblique, 2 mm. deep at the suture. There is no girdle. 

 Suture impressed, irregular. Color buckthorn-brown. Bands three, one ending at 

 lower edge of fissure, one at periphery and one at the base. Aperture ovate, yellowish- 

 white within. Columella white, broadly-rounded, having a heavy deposit of its material 

 at the fissure. Outer lip firm, sharp-edged, forming a sinuous line with the fissure. 



Operculum: Ovate, thick, dark reddish-brown, measuring 3 1-2 mm. diameter 

 by 5 1-2 mm. altitude. The left margin is thickened, smooth-edged; right margin 

 broadly curved, broken and worn; basal margin rounded, eroded close to the polar 

 point. Whorls three, the first two regularly and tightly coiled. Growth lines fine to 

 coarse. 



Measurements : Altitude, 15 mm. ; diameter, 10 mm. Aperture, base to suture, 

 10 1-2 mm.; base to lowest point of fissure, 6 mm.; diameter 4 1-4 mm. 



Type Locality : Coosa River, Cedar Island, Chilton County, Alabama. 



Type in Museum of Zoology, University of Michigan ; paratypes in Alabama 

 Museum of Natural History. 



This species appears to be a derivative of C incisum Lea which has 

 developed fixed, uniform specific characteristics. It is small for the genus 

 and varies much more in the size of mature specimens than in form. The 

 largest shell measures 17 x 11 mm. It is from Weduska Shoals. No folds 

 appear upon any of the material. Growth lines are usually fine, but some- 

 times coarse and roughened near the shoulder. The waving transverse 

 lines are strikingly beautiful in a few specimens which have not suffered a 

 great amount of wear. 



The fissure varies slightly in depth, but in none is it deeper than 3^ 

 mm. It is frequently only a sinuous indentation. The girdle is absent 

 in all the specimens examined. 



Color is sometimes as dark as burnt-umber. Of 65 shells, 51 have bands, 

 the formula in all being exactly that of the type. 



Very young shells are smooth, lacking carinas, plicae and all other sculp- 

 ture except microscopic growth lines. The base is obtusely angulate. The 

 sinus is well-marked, slightly oblique. The fissure, about 3^ mm. deep, 

 begins with the fourth whorl and is hardly more than a sinuous, inturned 

 part of the outer lip. Eight whorls are indicated for full-grown individuals. 



The known localities for zvalkeri are Weduska Shoals, Shelby County; 

 The Bar, Cedar Island, Higgin's Ferry and Duncan's Riffle, Chilton County ; 

 Butting Ram Shoals, Coosa County. One of Mr. Smith's labels carries 

 the note, "swift water." 



