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 

 {Savagassum). They comprise a series of Nudibranchs, 

 some of which are peculiar to the locality, a Tsenio- 

 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 Tsenioglossate 

 genera Potamides. Certain of the Paludestrinidse, 

 Assemania, Tmncatella, the Pulmonate Chilinia, and 

 Pelecypods such as Iphigenia and Scrobicularia. 



The true freshwater forms consist solely of Gastro- 



