t> THE NAUTILUS. 



Lymnsea petoskeyensis n. sp., PI. I, fig. 3, 5-7. 



Shell elongate oval, acutely conic, perforate; thin, pale horn- 

 color, almost white, translucent ; whorls 6, regularly increasing, con- 

 vex, with a well impressed suture ; spire elongated, acutely conical, 

 apical whorl minute ; body whorl somewhat inflated, elongate oval ; 

 lines of growth fine and regular, cut by numerous very fine revolv- 

 ing, spiral lines, surface more or less malleated ; aperture oval, sub- 

 angulate above and rounded below, slightly more than one-half the 

 entire length of the shell ; lip thin and sharp ; columella nearly 

 straight without any fold, inner lip expanded and reflected over the 

 round deep umbilicus and continued as a thick white callus over the 

 parietal wall ; where this callus passes over the umbilicus toward the 

 basal margin it is abruptly depressed into the umbilical opening, 

 forming a well marked furrow between the columella and the parietal 

 wall, and giving the appearance of a twist to the face of the columellar 

 enlargement, but the columella itself is scarcely affected by it; the 

 axis is large for the size of the shell, without any trace of a fold, 

 and nearly cylindrical, the base of the preceding whorl abruptly 

 flattened around the insertion of the upper end of the pillar. 



Alt. (Fig. 5) 23.5, diam. 11.25, ap. length 13, width 8 mm. 



Alt. (Fig. 6) 24.5, diam. 11, ap. length 13.5, width 7.5 mm. 



Alt. (Fig. 7) 25, diam. 10.5, ap. length 12, width 7 mm. 



Types (No. 14347 coll. Walker) from a small spring-brook flow- 

 ing into Little Traverse Bay, near Petoskey, Mich. Cotypes in the 

 collections of the Philadelphia Academy and the Chicago Academy 

 of Sciences. 



This species was at first supposed to be a very thin, fragile form 

 of the elongate variety of L. catascopium, characteristic of the lake 

 region. But upon cutting into the shell, the peculiar shape of the 

 axis forbade its reference to that species. 



Under Dr. Ball's arrangement of Lymncea (Harr. Exp. XIII, p. 

 64) it would belong to the section Galba. Compared with L. desi- 

 diosa Say, (Fig. 4) the axis of petoskeyensis (Fig. 3) is proportion- 

 ately much larger, more elongated and more cylindrical, but the 

 general features of both are the same. The peculiar contraction of 

 the base of the whorl around the upper end of the pillar, so remark- 

 ably developed in petoskeyensis, is present, but not at all marked, in 

 desidiosa. The umbilicus in petoskeyensis is round and deep, and is 

 more conspicuous in the immature shells, as the expansion of the 

 broadly reflected columella nearly covers it in the adult. 



