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 (161/2-19) 

 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 22-23%, diam. 5%-6 mm. 



H. semisculpta, no. 15. 



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



H. codhuilensiSj no. 16. 



