THE CYOLOPHORTD LAND M01.LUSK9 OF AMEKTCA. 237 



THE GROUP OF APEROSTOMA (APEROSTOMA) GIGANTEUM 



Posterior angle of aperture forming a slight angle, bearing a weakly 

 impressed median groove. 



APEROSTOMA (APEROSTOMA) CONFUSUM (Sykes) 



Plate 33, Figures 1-3 



1846. Cyclostoma giganteum Pfeiffee, Martini-Chemnitz Conchylien Cabinet, 

 vol. 1, sect. 19, p. 11, pi. 1, figs. 11-14 (not C. giganteum Reeve, Con- 

 chologia systematica, vol. 2, p. 99, pi. 184, fig. 17, 1842). 



1901. Aperostoma confusum Sykes, Joum. Malac, vol. 8, p. 106, pi. 10, fig. 2. 



1923. Poteria (Neocyclotus) confusum H. B. Baker, Dec. Pap. Mus. Zool. 

 Univ. Michigan, No. 137, p. 42. 



Shell very large, helicoid. Our specimen of this species, unfor- 

 tunately, is sadly denuded. However, it shows that there is a broad 

 dark chestnut-colored band below the periphery and apparently a 

 narrow light zone immediately above this; interior of aperture pale 

 buff. The nucleus consists of about 2.4 small, well-rounded, smooth 

 turns. The postnuclear whorls are marked by decidedly retractively 

 slanting, rather irregular, closely approximated axial riblets, which 

 on the last half of the last turn become confluent incremental ele- 

 ments. The summit of the whorls is appressed, and there is a slight 

 concavity on the last turn below the summit. Suture well impressed. 

 Periphery strongly rounded. Base narrowly umbilicated and marked 

 by the continuation of the axial sculpture, which does not increase 

 in differentiation on the umbilical wall. Aperture circular, oblique, 

 with a small angulation bearing a central groove at the posterior 

 angle; peristome simple; outer lip thin; the inner thickened and 

 slightly reflected. The parietal wall is covered by a thick callus, 

 which renders the peritreme complete. This callus is not in a straight 

 line with the columella, but forms a little oval sinus. 



The specimen that we believe belongs here, U.S.N.M. No. 341766, 

 was collected by A. E. Heighway at Acandi, a small settlement in the 

 Gulf of Atrato, on the boundary between Colombia and Panama. 

 It has 5.3 whorls and measures : Height, 34.2 mm. ; greater diameter, 

 51.5 mm. ; lesser diameter, 38.2 mm. Height of aperture, 20.5 mm. ; 

 diameter, 24.8 mm. 



This species is differentiated from the rest of the giganteum group 

 by having the spire much more elevated. 



APEROSTOMA (APEROSTOMA) GIGANTEUM (Reeve) 

 PiATE 33, Figures 7-9 



1842. Cyclostoma giganteum Reeve, Conchologia systematica, vol. 2, p. 99, pi. 



184, fig. 17. 



1843. Cyclostoma giganteum Sowerby, Thesaurus conchyliorum, p. 92, pi. 23, 



figs. 8, 9. 

 428349 — 42 16 



