130 SUCCINIC. 



Genus HELISIGA, Lesson. 



Eye-peduncles short, cylindrical, swollen at the base; 

 tentacles short, rudimentary. 



Shell ovate, ventricose; spire very short; aperture wide, 

 patulous ; peristome simple, acute. 



Ex. H. Sanctee-Helense, Lesson, pi. 73, fig. 4. Shell, 

 H. fragilis, Eycloux and Souleyet, fig. 4, a. 



There is a group of Succinece from the Society and Sand- 

 wich Islands, which agrees in the form of the shell with 

 this genus, hut the mantle-margin is extended and covers 

 the outer lip. The same is probably the case in the Helisiya 

 of South Americ?. 



Species of Helisiya. 



aperta, Lea. rotundata, Gould. 



cepulla, Gould. Sanctse Helena, Lesson. 



fragilis, Eyd. and Sold. 



Genus OMALONYX, D'Orbigny. 



Mantle forming a thickened border round the shell, cover- 

 ing the margins ; pulmonary orifice under the right border 

 of the mantle towards tbe middle part. 



Shell oval, depressed, unguiform; spire none; aperture 

 wide, open. 



Syn. Pelta, Beck, not Quatrefages. 



Ex. O. unguis, D'Orbigny, pi. 73, fig. 5. Shell, 0. 

 unguis, fig. 5, a. 



The Omalonyx is found in the marshes of Meridional 

 America ; it crawls rapidly and is more aquatic than other 

 Succinea, dying when removed from the vicinity of the water. 



