192 DRYM^EUS, SOUTH AMERICAN. 



p. 162, for B. subsemiclausus. Mormus ALB., DieHel., 1860, p. 216,. 

 type B.papyraceus. 



The passage of DrymoBUS with expanded or reflexed peristome in- 

 to Mesembrinus with it simple and acute, is a gradual one, and many 

 species are arbitrarily assigned to one or the other section, although 

 the more typical manifestations of each group are conspicuously dis- 

 tinct. In some cases it is easy to see that species of both groups in- 

 habiting one region are of common origin, and more nearly related 

 to each other than to species of their respective sections in distant 

 localities. Mesembrinus seems to be the polyphyletic group of the 

 two. This being the case, a geographic grouping into 1, South 

 American, 2, Antillean and 3, North American species has been 

 adopted, although this, too, has its disadvantages. 



I. South American species. 



Group of D. incequalis. 



D. ABYSSORUM (d'Orbigny). PI. 37, figs. 3, 4. 



Shell oblong, ventricose, quite thin, appearing smooth or even 

 shining to the naked eye, but under the lens showing wrinkles and 

 irregular transverse stri?e ; dirty white throughout, with wide longi- 

 tudinal bands of purple-brown, unequal in width ; spire lengthened, 

 a little in Hated, the apex acute, composed of 7 flattened whorls sep- 

 arated by a little-impressed suture. Aperture oblong, not oblique ; 

 peristome wide, somewhat reflexed, thin, whitish ; columella percep- 

 tibly swollen, umbilicus not widely open. Alt. 50, diam. 25 mill. 

 (Orb.). 



Along the Rio Grande betiveen Valle Grande and Pescado in the 

 most rugged places, in the easternmost foot-hills of the Bolivian Andes, 

 main/y at a place called Pampa Ruis, under stones and cacti (Orb.) ; 

 San Lorenzo, prov. Jujuy, Argentina (Borelli). 



Helix abyssorum ORB., Mag. de Zool., 1835, p. 17. Bulimus abys- 

 sorum ORB., Voy. Amer. Merid., p. 308, pi. 39, f. 7, 8. PFR., Monogr. 

 ii, p. 46; iii, p. 331; iv, p. 393; vi, p. 39. ?KEEVE, Conch. Icon., pL 

 36, f. 213. Bidimulas abyssorum BECK, Index Moll., p. 64. AN- 

 CEY, Boll. Mus. Zool. ed Anat. Comp. R. Univ. Torino, xii, No. 309,. 

 p. 12 (1897). 



There is also a uniform white form of this species. Ancey con- 

 siders B. abyssorum, hygrohylwus and marmarinus as varying forms of 

 one species. An adult specimen collected by Dr. Borelli measures : 



