.VALLONIA. 253 



horn colored, with rather regularly set membranous ribs and finer 

 stride between them ; nucleus with fine revolving lines. Whorls 3i, 

 slightly flattened above and below the periphery, at the circumfer- 

 ence somewhat angular, with a deep suture, rather rapidly increas- 

 ing ; the last whorl expanding to the aperture, shortly and moder- 

 ately descending in front, more so at the suture than on the back. 

 Aperture rather oblique and inclined, almost circular, a little flattened 

 above, ends of margin much approximating, slightly protracted and 

 auricled, connected by a thin callus; peristome strongly and abruptly 

 everted, with a strong white lip. 



Alt. 1*1, greater diam. 2'5, lesser 2*1 mill. 



Circumboreal ; Europe, eastern and northern Asia to the Amur; 

 northern Africa ; in North America it has about the same range as 

 pulchella, but seems to be not so common. 



Hel. costata MULLEK, Verm. Hist. If, 1774, p. 31, EOSSM., Icon. 

 f.439; DUPUY, Hist. 1848, p. 162, t. 7, f. 4. WESTERLUND, Fauna 

 I, p. 14, and of other authors. Hel. or Vail, pulchella var. costata 

 of authors.- - F. rosalia Risso, 1826. 



Jaw rather curved. Radula with 69-72 transverse rows of 27 or 

 29 teeth ; 5 perfect laterals, on which the cusps are comparatively 

 small, especially the ectodonts. On the marginals the 6-7 cusps 

 stand immediately on the plate, not on a common base. 



V. costata has, by most authors been regarded as a variety of pul- 

 chella ; but it is sufficiently different to be distinguished at first sight ; 

 the less elevated spire, with the deeper suture, the whorls not so well 

 rounded, the last descending in front, the more oblique and more 

 circular aperture with approximate margins, characterize quite a 

 different shell, even if we omit mention of the most conspicuous 

 feature of fresh specimens, the membranous ribs, which this species 

 has in common with a number of other forms of this group and the 

 next. Between each pair of these ribs there are 3 to 6 finer strife ; 

 and under the microscope very fine lines are seen, nearly parallel to 

 the lines of growth at the suture, while on the periphery they are 

 variously intercrossing as shown in fig. 27. The nucleus, or 

 embryonal whorls, bear a number of microscopic revolving line?, in 

 this as well as in the other species of this group. 



V. costata is rather variable. The largest measure 2'7, the small- 

 est 2'1 mill, greater diameter. Always the sutural part of the last 

 whorl descends more to the aperture than the back or peripheral 

 part of the whorl, and sometimes the latter keeps exactly in the 



