556 SHELLS AND SHELL-FISH. PART II. 



Fissurella. Apex nearly central ; aperture large, oval 

 simple. 

 F. picta. Sow. Gen. f. 1. gigas. Sw. Sp. Nov. 



Machrochisma Sw. Aperture very large, oblong, placed 

 closed to the margin. 



M. hiatula Sw. Sow. Gen. f. 5. 



Clypidella Sw. One extremity of the shell, near the 

 perforation, slightly raised, truncated, and sub-emar- 

 ginate. 



C. pustula. Sow. Gen. f. 3. 



Fissuridea Sw. Sub-conical, cap-shaped ; the summit 

 close to the posterior margin ; the perforation narrow. 

 T. pileus. Sw. Sp. Nov. 



EMARGINULA Lam. Either patelliform or cap-shaped ; 

 the apex near the posterior end ; the anterior margin 

 or summit with a very narrow slit or fissure. 



Emarginula Lam. Cap-shaped; fissure on the an- 

 terior margin. 



E. reticulata. Sow. Gen. fissura. Sow. Man. f. 241. 

 5. elegans. Ib. f. 4. 



Cemoria Leach. Cap-shaped ; fissure down the centre 

 of the apex. 



C. Fleming!!. Sow. Man. f. 244. 



Rimula Def. Cap-shaped ; fissure long, in the middle 

 of the shell ; margin entire. 



R. Blainvillii. Sow. Gen. f. 243. 



Parmophorus Lam. Patelliform ; fissure assuming the 

 form of an anterior sinus, scarcely perceptible on 

 the anterior margin. 

 P. elongatus. Sow. Gen. f. 1. breviculus. Sow. Gen. f. 2. 



Hemitoma Sw. Patelliform ; the fissure not cut through 



the shell, but merely forming an internal groove. 



H. tricostata Sw. Sow. Gen. f. 6. 



HIPPONYX. Shell cap-shaped ; the apex sometimes 



