THE NAUTILUS. 103 



III. Apex of shell subspiral, not removed ; or if it be removed, the 

 hole-callus inside is truncated or has a pit behind ; or 

 there is a plate inside, as in Crepidula. Central tooth of 

 radula wide, Emarginulin^. 



Subfamily I. Fissurellince. 



There is only one genus, Fissurella. 



A. Summit of the shell near the middle ; basal margins level, not 



elevated at the ends, Subgenus Fissurella. 



a. Edge of shell not crenulated, dark-bordered inside-true, 



Section Fissurella. 

 h. Edge of shell crenulated, not dark bordered inside, section, 



Section Cremides. 



B. Shell flattened, shield-shaped, the narrow hole in front of the 



middle; ends of shell elevated, Subgenus Clypidella. 



Genus 1. Fissurella Brug. 



1. F. volcano Reeve. Santa Cruz, Cal., southward. 



(Section Cremides H. & A. Ad.) 



2. F. barbadensis Gmelin. Charlotte Harbor, Fla., southward. 

 One of the commonest West Indian shells, easily known by its 



almost circular perforation. 



3. F. nodosa Born. Florida Keys. 

 The ribs are nodular, orifice oblong. 



(Subgenus Clypidella Swains.) 



4. F.pustula Lam. Cape Lookout, southward. 



5. F. fascicularis Lam. Florida Keys. 



The anatomy of these is not known. Collectors should preserve 

 specimens of the animal. 



Subfamily II. Fissurellidince. 



A. Mantle entirely or nearly covering the shell ; hole large. 



a. Edges of shell nearly level, beautifully crenulated, 



Lucapina. 



b. Edges of shell elevated at each end, blunt at the sides, not 



crenulated, Megatebennus. 



B. Mantle not enveloping the shell. 



a. Perforation about central, the shape of the shell, 



Lucapinella. 



