14 TROCHUS. 



circular, peristome simj^le, acute, the margins apjiroaching ; columella 

 arcuate, simple, thin. Type, M. heliciua Fab. 



Arctic and Subarctic Seas. 



Subgenns Bathymophila Dall, 1882. 



Shell when immature like Margarita ; adult with a broad flattened 

 columella, which has a blunt tooth, rough or granulated, at its end. 

 Type, M. euspira Dall. 



Genus SOLARIELLA Searles Wood. 1842. 



Shell umbilicated, conical ; Avhorls with spiral granose lirse ; 

 umbilicus with carinated margin. Type, S. maculatum AVood. 



Subgenus Turoicula Dall, 1881. 



Shell globose] y conical, white, thin ; umbilicus reduced to a chink 

 under the thin callus of the upper part of the pillar lip ; mouth 

 rounded rectangular, margins all thin; columella concave; outer 

 surface with tuberculose ridges. Type, M. imperialis Dall. Cuba. 

 (This group is placed under Calliostoma by Fischer. Its position is 

 problematical until the structural details are known.) 



Genus CALLIOSTOMA Swainson, 1840. 

 Shell imperforate or rarely umbilicate, conical, rather thin ; whorls 

 smooth, spirally ridged or granular, the last angulated at the periph- 

 ery; aperture quadrangular; columella simple, usually ending an- 

 teriorly in a slight tooth. Type, C. zizi2:)hiuum L. 



Subgenus Calliostoma (restricted.) 

 Axis imperforate, its lower termination covered by a slight ex- 

 pansion of the columella. The sections of this subgenus are given 

 in the text. 



Subgenus Eutroohus A. Adams, 1863. 



Shell umbilicated ; columella ending in a point or tooth below. 

 Type, C. javanicus Lam. 



Genus TURCICA H. and A. Adams, 1854. 

 Shell conoidal, thin, subdiaphanous, imperforate ; whorls with 

 transverse series of granules, the last rounded on the periphery; 

 columella thick, spirally twisted posteriorly, ending anteriorly in an 

 obtuse, prominent jjoint ; outer lip thin, simple, acute (H. & A. Ad.) 

 Type, T. monilifera A. Ad. Australia. 



