152 AMERICAN JOURNAL 



situated the group of Central American lakes. The river San 

 Juan, which discharges the surplus water of Lake Nicaragua, 

 flows through a transverse valley to the Atlantic, at Grey town. 



All the species enumerated in this paper were collected to the 

 east of the Pacific coa'st range. 



The land shells which are limited to the volcanic country are 

 Helix griseola, Glandina rosea, Bulimus iinieolor, B. discrepans, 

 Helicina rostrata, &c. ; Bulimus castus, Helix Parheri, H. cce^ 

 coides, Tcbennophorus auratiis, Kryniekia Americana^ and Heli- 

 cina denticulata, are confined to the mountain forests. 



All the Unionidm, Ampullarioe, Tryonice, Planorhes, Neritinoe 

 and Pltysce were collected in the basin of the Lake Nicaragua 

 and tlie San Juan ; Melania, Amnicola and Sphcerium being 

 confined to the Chontales District. 



11. Enumeration of the Species. 



I would acknowledge here that I am under great obligation to 

 Mr. Tryon for the majority of the specific determinations. The 

 species enclosed within brackets are extra-limital. Excepting 

 Tebennophorus auratus, all the species have been deposited in 

 the Museum of the Society. 



1. POMUS PYRUM, Phil. 



Occurs in the river San Juan, its tributaries and creeks ; in 

 the river Panaloya, and in Lake Nicaragua at Grenada. 



This species usually lives on the muddy bottom of still-water 

 courses, and though sometimes observed floating on the surface 

 of the water, yet if surprised in that position it immediately 

 sinks to the bottom. The eggs are deposited in semi-pyriform 

 masses on the stems of Oyperus and small trees growing in or at 

 the margin of the water ; the capsules are shelly, greenish-white, 

 numerous, and by compression hexagonal. In every instance 

 the aggregation of capsules was placed from a few inches to two 

 and three feet above the highest level of the water, and it is be- 

 yond a doubt that the parent mollusc leaves the water for the 

 purpose of nidification. The species of Pomiis observed by me 

 in South America have the same habit, whilst Marissa deposits 

 its gelatinous egg-masses always below the surface of the water. 



2. Melania Gassiesii, Reeve. 



Lives on stones in the rocky beds of the clear, rapid-flowing 

 streams taking their rise in the Chontales Mountains. The 

 species is confined to Central America. 



