PRESENT HISTORY AND DISTRIBUTION 57 
of many species, also pass their lives far from land, 
with the larger number of the Cuttlefishes. 
A more peculiar pelagic assemblage is afforded by 
«those molluscs that dwell among the Gulfweed 
(Saragassum). They comprise a series of Nudibranchs, 
some of which are peculiar to the locality, a Tzenio- 
glossate (Litiopa), and two or three Limpets (Helcion 
and Lepeta). 
Just as some representatives of the race have crept 
down into the depths, so others have ascended the 
rivers, and there become accustomed to a freshwater 
existence. 
In the brackish water near the mouths of rivers, 
and occasionally for long distances up the rivers 
themselves, some stragglers from the marine host 
are to be met with, such as Cerithium, Littorina, 
Nerita, Mytilus, Cardium, Macoma, Mya, etc., where 
they may be found dwelling side by side with fresh- 
water kinds. The genus Theodoxis, of course, has 
both marine and freshwater species, while there are 
freshwater representatives of the Ark-Shells, such as 
Scaphula pinna, of Volsella, and of the Piddocks, 
Pholas rivicola, and others. 
There is, however, a distinct brackish-water group, 
including Gastropods belonging to the Tzenioglossate 
genera Potamides. Certain of the Paludestrinide, 
Assemania, Truncatella, the Pulmonate Chilinia, and 
Pelecypods such as Iphigenia and Scrobicularia. 
The true freshwater forms consist solely of Gastro- 
