50 STERKIA. 



lacking, it may perhaps be inferred that the modem Sterkise 

 are diminutive descendants of an early Tertiary Ptychalaa- 

 like stock of Holarctic distribution. 



The typical species of the genus are remarkably specialized 

 in teeth. The section Metasterkia contains more primitive 

 species. 



The living animal has not been observed. It will probably 

 be found to lack inferior tentacles, if our estimate of its 

 affinities is correct. 



The Eastern or Antillean herd is widely remote from that 

 of Lower California, yet we know so little of the minute 

 shells of tropical America that part of this separation may 

 be due to the deficiencies of our Mexican collections. The 

 genus will probably be found widely spread in the West 

 Indies and shores of the Caribbean Sea. 



Key to species of Sterkia. 



a\ Shell distinctly rib-striate; lamellae and plicas large; the 

 inner end of the columellar lamella bent vertically down- 

 ward (Section Sterkia, proper). 



6 1 . Columellar lamella forming a large, vertical plate, 

 visible in the aperture ; both palatal folds visible in 

 a front view. Lower California. 8. calamitosa, no. 1. 



6 2 . Vertical part of the columellar lamella not conspic- 

 uous ; lower palatal fold not visible in a front view ; 

 upper palatal fold very long; base humped. South- 

 ern and Lower California. 8. hemphilli, no. 2. 



a~. Shell nearly smooth ; palatal folds not very large, all vis- 

 ible in a front view; columellar lamella turned upward 

 within or horizontal (Section Metasterkia, type 8. antil- 

 lensis). 



ft 1 . Columellar lamella median, short, entering horizon- 

 tally ; five or more Avhorls. California. 



8. Clementina, no. 3. 



ft 2 . Columellar lamella situated high, ascending within ; 

 Florida to Guyana. 



c 1 . Palatal folds small and short. 



