- HOLOSPIRA. 89 



Subgenus HAPLOCION Pilsbry, 1902. 



Internal axis smooth, the interior of the whorls without 

 plaits or lamellae; latter part of the last whorl straightened, 

 not sinuous; the aperture not obstructed by folds or promi- 

 nences of any kind. Type H. pasonis Dall. (haploos, sim- 

 ple; kion, pillar.) 



This subgenus is closely related to Bostrichocentrum and 

 Haplostemma, differing in the simplicity of the pillar within* 

 Metastoma is distinct by peculiar modification of the last 

 whorl. There are three groups of species. 



I. Internal axis large, one-third the diam. of the shell; 

 shell smooth, brown, composed of many (16^-19) 

 closely coiled whorls. H. fusca, no. 20. 



II. Internal axis smaller, one-sixth to one-fourth the diam. 

 of shell; shell small (length 9-14 mm. in known species), 

 brownish, compactly coiled, the whorls short, 11 to 13 

 in number, sculptured with ribs or riblets. 



1. Internal axis swollen above, tapering below, about 

 one-fourth the diam. of shell; nepionic whorls 

 strongly angular, the rest coarsely ribbed; length 

 11-14 mm. H. minima, no. 19. 



2. Internal axis smaller and of about uniform calibre ; 

 nepionic whorls not angular. 



a. Whorls of the cone flattened, and usually 

 angular at the lower edge, finely and closely 

 ribbed; length 9-11.3 mm. 



H. remondi, no. 18. 



&. Whorls of the cone convex, more strongly and 

 distantly ribbed; length 11, diam. 3.7 mm. 



H. crossei, no. 17. 



III. Internal axis very slender below, slightly wider above; 

 shell rather large (length 22-29 mm. in known species), 

 white, the individual whorls high, Iiy 2 -15 in number, 

 the last one or two more coarsely ribbed than the inter- 

 mediate whorls. 



1. Whorls 14-15 ; length 22-23y 2 , diam. 5y 2 -6 mm. 



H. semisculpta, no. 15.. 



2. Whorls 12 ; length 29, diam. 7 mm. 



H. coahuilensis, no. 16. 



