192 BULUETIIN 181, UTSPITEiD STATES NlATTOiNlAiL MUSIEUM 



and marked by the continuation of the axial riblets, which become 

 much strengthened on the umbilical wall. Aperture almost circular, 

 excepting the siphonal projection at the posterior angle; peristome 

 thickened, almost double on the columellar lip, thin on the outer lip, 

 where it is projected upward over the periphery of the preceding 

 turn and backward deflected with a narrow sinus between it and the 

 preceding turn. 



The type, U.S.N.M. No. 307480, comes from Brazil. It has 4.7 

 whorls and measures : Height, 23.5 mm. ; greater diameter, 45.8 mm. ; 

 lesser diameter, 32.6 mm. Height of aperture, 16.7 mm.; diameter, 

 20.5 mm. 



This is the most depressed of all the members we have seen, which 

 feature alone readily distinguishes it from the other species. 



APEROSTOMA (INCIDOSTOMA) INCOMPTUM (Sowerby) 



Plate 26, Figukes 4-6 



1850. Cyclostoma incomptum Sowerbt, Thesaurus conchyliorum, vol. 1, suppl., 

 p. 160*, pi. 31A, figs. 298-299. 



1852. Pterocyclos incomptus Pfeiffee, Monographia pneumonopomorum viven- 



tium, vol. 1, p. 42. 



1853. Pterocyclos incompUis Pfeiffeji, Martini-Chemnitz Conchylien Cabinet, 



vol. 1, sect. 19, p. 235, pi. 31, figs. 1-2. 

 1897. Neocyclotus (Neocyclottis) incomptus Kobelt and Mollendokff, Nachrb. 



deutschen malak. Ges., vol. 29, p. 137. 

 1923. Poteria (Neocyclotus) incomptus H. B. Baker, Occ. Pap. Mus. Zool. 



Univ. Michigan, No. 137, p. 30. 



Shell almost planorboid, decidedly depressed. The specimen be- 

 fore us has lost its periostracum and is white, with a brownish tinge 

 on all but the last whorl. (Sowerby says of the type that it is cov- 

 ered with an olivaceous epidermis.) The nucleus consists of a little 

 more than one turn, which is well rounded and smooth. The post- 

 nuclear whorls are strongly rounded and marked by somewhat irreg- 

 ular, broad, low, rounded, retractively curved axial riblets, which 

 are wider than the spaces that separate them. Suture deeply im- 

 pressed, almost channeled. Periphery with a faint indication of a 

 thread. Base broadly openly umbilicated, showing all the whorls 

 in the umbilicus. The base is strongly rounded and marked like the 

 spire. Aperture circular with a strong auricle at the posterior angle, 

 which is reflected and almost forms a siphon. Outer lip thin; the 

 inner lip is slightly thickened. 



The specimen described, U.S.N.M. No. 515940, has 4.4 whorls and 

 measures : Height 17.8 mm. ; greater diameter, 36.5 mm. ; lesser diam- 

 eter, 26.0 mm. Height of aperture, 14.6 mm.; diameter, 16.5 mm. 



The small size and strongly reflected outer lip should readily dis- 

 tinguish this from the other smaller species. The shell described 



