158 FISSURELLA. 



F. OBOVALIS Lesson. 



This Fissurella is elongated-oboval, rounded posteriorly, contracted 

 in front, a little produced in front and behind so that when placed 

 upon a horizontal plane the extremities alone support it. Fissure 

 oblong, its sides straight, thick, 4-toothed ; the fissure placed in 

 front of the middle. Outer surface having close radiating ribs, 

 decussated by concentric striae. It is olive-brown, often encrusted. 

 Inside porcelain-white. Edge entire, blackish, a little oblique. 

 The horse-shoe-shaped muscle-impression is strongly marked. (Less 

 I. c.) Length 21, breadth in front 8, behind 18, alt. 7 lines. 



Province of Conception, Chili. 



Section CREMIDES H. & A. Adams. 



Cremides ADS. Genera Rec. Moll, i, p. 446, 1858. 



Fissurella with the orifice near the middle, the outer surface radi- 

 ately ribbed or striated, the inside without a dark marginal border, 

 and more or less crenulated on the edge, group in this section. 

 They inhabit tropical and subtropical coasts of America, with a 

 few species from South Africa and the Mediterranean. 



The species, in the following pages are grouped geographically 

 a grouping probably as natural as any other that could be devised 

 for such variable shells. 



Species from the West Coast of the Americas. 



F. VIRESCENS Sowerby. PL 31, figs. 15, 16, 17. 



Shell oval, elevated, conical ; the orifice about in the middle, 

 large, contracted in the middle, its sides more or less flaring out- 

 ward. 



The shell is very solid and strong, oval, or a little narrower 

 anteriorly, elevated, sculptured with numerous fine unequal (alter- 

 nately larger and smaller) radiating riblets. Color uniform greenish 

 gray without rays. Inside green, smooth, the callus around the 

 perforation flat ; border finely crenulated. 



Length 50; breadth 38, alt. 18 mill. 



Panama to Mazatlan. 



F. crassa SOWB. Genera of Shells, (Cephala), Fissurella f. 2. F. 

 nigropunctata SOWB. P. Z. S. 1834, p. 125 ; Conch. Illust., f. 51. 

 REEVE, Conch. Icon. f. 8. Sowb. Thes. p. 188, f. 33. F. virescens 

 SOWB. P. Z. S. 1834, p. 125. REEVE, Conch. Icon. f. 12. SOWB., 



