UROCOPTIS, S. G. IDIOSTEMMA. 165 



which a callous band or double cord bears oblique nodes or 

 riblets. The external ornamentation and the general shape 

 are wonderfully varied, and in former artificial classifications 

 the species have been widely scattered. They are illustrated 

 on plates 44, 45, and the upper part of pi. 46. 



Key to Species of Idiostemma. 



I. Axis armed with pairs of hooks, at least above. 



1. Hooks swollen; shell strongly ribbed. 



U. uncata, no. 39. 



2. Hooks compressed; shell smooth, with subsutural 

 beads. U. perlata, no. 40. 



3. Hooks above only ; shell smooth throughout. 



a. Diam. about 4.5 mm., whorls about 12; flat 

 axial nodes in the median whorls. 



U. laevigata, no. 41. 



&. Diam. about 3 mm., whorls about 14; axis 

 with 2 thick lamellae below. 



U. pilotensis, no. 42. 



II. Axis thickened by a callous, obliquely ribbed band, or 

 a double spiral cord bearing oblique nodes. 



1. Shell ribbed. 



a. Axis encircled by a stout double cord, the 

 sulcus between obliquely ribbed (pi. 45, fig. 

 53) ; shell irregularly ribbed below. 



U. geminata, no. 43. 



&. Axis bearing compressed, sigmoid, wide- 

 spaced ribs, with concave intervals (pi. 45, 

 fig. 47) ; exterior with regular, rather narrow 

 ribs. U. intusmalleata, no. 44. 



c. Axis bearing a spiral callous band, thickened 

 above and below in each whorl, and obliquely 

 ribbed (pi. 45, fig. 41). U. fastigiata, no. 45. 



2. Shell ribless, sometimes with sutural and basal 

 nodules ; very long and tapering. 



U. lateralis, no. 46. 



The smooth forms, lavigata, pilotensis, perlata and lateralis, 

 are in all probability descendants of ribbed species. All the 



