OF MOLLUSCA. 65 



a long time out of water when the marshes dry up. They afford 

 nourishment to the marsh birds, especially the large Rails. 



§ Siphon not produced. Operculum horny, with shelly internal 

 coat. Peristome thickened. 



1. Pachystoma. 



Shell subglobose ; spire short. Peristome continuous, thickened 

 on the edge within ; nucleus subcentral. Operculum horny, with a 

 shelly internal coat. — Adams, Gen. Moll. t. 37. f. 1 a, b. 



1. P. fasciata, t. 47. f. 3 ; Adams, Gen. Moll. t. 37. f. 1. 



2. P. Cebensis, t. 47. f. 1, 2 & 4. 



2. ASOLENE. 



Shell subglobose ; spire short ; aperture oval ; inner lip slightly 

 thickened ; outer thin ? Operculum horny, with an internal shelly 

 coat ; nucleus lateral, anterior ; siphon not exposed. 



1. A. Platse, t. 47*. f. 4. 



Asolene Plata is found in small marshes on the sands near the 

 La Plata, at the roots of water plants ; its movements are very slow, 

 and it seldom comes out of its shell. 



§§ Siphon of mantle elongate, produced. Operculum horny, simple. 

 Peristome thin. 



3. Marisa. 



Shell discoidal, largely umbilicated. Peristome simple. Oper- 

 culum horny {Adams, Gen. Moll. t. 37. f. 3). Siphon elongate. 



1. M. cornu-arietis. Ampullaria c, Lamk. ; Adams, Gen. Moll, 

 t. 37. f. 3. 



4. Ampullaria. 



Shell subglobose, perforated ; mouth large. Peristome simple, 

 thin. Operculum horny, dextral {Adams, Gen. Moll. f. 2 a, b) 

 (fig. 35). Siphon elongate. 



1. A. zonata, t. 47*. f. 2. Fig. 35.— Operculum of 



2. A. Rossii, t. 47*. f. 1. Ampullaria. 



3. A. scalaris, t. 47**. f . 2 ; Adams, Gen. Moll. 



t. 37. f. 2. 



4. A. Ureus, Troschel, Wiegm. Arch. 1845, 197. 



t. 8 (anat. f. 3, teeth). 



5. A. canaliculata, t. 47**. f. 1. 



6. A. insularum, t. 117. f. 1. 



7. A. , t. 117. f. 2. 



Lingual membrane of Ampullaria ? linear elongate. Teeth 



3 * 1 • 3 ; central broad, short ; apex slightly recurved, with a strong 

 central tooth and a small toothlet on each side ; inner lateral tooth 



