160 HEMITOMA. 



these sub-divisions, for the sake of presenting the reader with 

 the principal variations to which the genus is subject. The 

 established genera will be characterized in their places. Fig. 254 

 to 281. 



HELIXARION. Fer. Vitrina, Drap. Differing from Helico- 

 limax in the structure of the animal. Fig. 262. 



HEMICARDIUM. Cuv. (r//-uaue, hemisus, half, Kapha, cardia, 

 heart.) Cardium Hemicardium, fig. 123**, and several similar 

 species. 



HEMICYCLA. Sw. A sub-genus of Helix. 



HEMICYCLONOSTA— see Cardilia. 



HEMICYCLOSTOMATA. Bl. The fourth family of Asiphonibran- 

 chiata, Bl. described as " more or less globular, thick, flattened 

 on the under side; spire very short; aperture large, semilunar, 

 entire; its outer edge hollowed; its inner or columellar edge 

 straight, sharp and septiform." This family answers to the 

 genus Nerita of Linnaeus, and to the family Neritacea of 

 Lamarck. It contains the genera Natica, Nerita, Neritina, and 

 Navicella. 



HEMIMACTRA. Sw. A sub-genus of Mactra, thus described : 

 " General form of Mactra ; but the cardinal teeth entirely want- 

 ing ; cartilage internal, central, in a large triangular cavity ; 

 lateral teeth 2, distinct, lateral, striated: connected to the Gly- 

 cimeri. H. gigantea, Lam. v. 472. No. 1. grandis, Stu. Sp. 

 Nov." Sw. p. 369. 



HEMIMITRA. Sw. A sub-genus of Paludomus, Sw. (Melanianse.) 



HEMIODON. Sw. A sub-genus of Anodon, described as having 

 " Tubercles or undulations on the hinge margin. H. undulatus, 

 purpurascens and areolata." 



HEMISINUS. Sw. A sub-genus of Melania, thus described : 

 " General shape of Melania ; but the base of the aperture is con- 

 tracted and emarginate; outer lip crenated. H. lineolata, Griff. 

 Cuv. xii. pi. 13. fig. 4." 



HEMITOMA. Sw. A sub-genus of Emarginula, thus described : 

 " Patelliform; the fissure not cut through the shell, but merely 



