HELIX-OXYCHONA. 129 



Key to species of Oxychona. 



L Shell trochiform, with conical or pyramidal spire, acute pe- 

 riphery, and flattened base ; apical whorls convex ; last two whorls plane 

 or plano-concave, microsco2)ically spirally striated. 



Umbilicus perforated 



Narrow, elevated ; outer lip not angled nor produced 



pilei/ormis. 

 Trochoidal ; outer lip angled, expanded rostrated 



zhorquinensis. 

 Umbilicus closed 



Outer lip above carina expanded 



Banded ; last whorl deflexed, pitted at carina lonchostoma. 



Unicolored ; last whorl scarcely deflexed, not pitted yyrina. 



Outer lip above carina not expanded bifasdata. 



II. Shell trochoidal or depressed, with conoidal or depressed spire, 

 carinated or rounded periphery and convex base; sutures moderately 

 impressed ; outer whorl convex above, not spirally striated. 



Umbilicus open costaricensis. 



Umbilicus imperforate 



Depressed, periphery angular ; diam. 27 mill, or more 



c/aillarmodi. 

 Conoidal, periphery rounded ; diam. 26 mill, or less 



trigonostoma. 



H. BIFASCIATA Burrow. PL 57, figs. 34, 35, 36, 37. 



Imperforate, trochiform, with straight or concave sided conical 

 spire, flattened base and acutely carinated periphery; thin, almost 

 opaque, shining, milky white or a little stained with rosy, unicolored 

 or spirally encircled by dark purplish-brown rather narrow bauds, 

 two or one on the upper surface of the last whorl, and one (rarely 

 two) near the outer edge of the base ; the bands of upper surface 

 stopping a distance short of the peristome, often reappearing again 

 behind it as two wider brown blotches in line with the bands. The 

 loAver whorls are seen to be obsoletely marked with fine spiral im- 

 pressed lines under a strong lens, and just above and below the 

 carina the whorl has an obsolete malleation, or pitted-crenate 

 character. The spire is elevated, with plano-concave outlines ; apex 

 blunt, embryonic whorl smooth, shining, rounded ; whorls about 7, 

 the first two quite convex with deeply impressed sutures ; the 

 following becoming flatter, separated by linear sutures ; last three 



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