VI.] INTRODUCTION. ci 



The same peculiarity _, but not of so permanent a cha- 

 racter, has been observed in the case of a freshwater 

 bivalve. The common pond-mussel [Anodonta cygnea) 

 is said to live in the river Trent at Bottesford in Lincoln- 

 shire, which is salt at high water. The fresh water, 

 being lighter, forms the upper stratum ; while the sea- 

 water covers the bed of the river inhabited by the Ano- 

 donta. A small Pectinibranch univalve (Hydrobia ulva) 

 inhabits indifferentlv salt and brackish water ; another 

 (H. ventrosa) frequents only the latter ; while a third 

 {H. similis) lives, in company with Bythinia tentaculata 

 and other freshwater univalves, in water which is nearly 

 fresh. The usual habitat of the genus Melampus (which 

 is Pulmonobranch) is the sea-shore ; but one species 

 occurs high up in estuaries, where the water is more 

 fresh than salt. M. Marcel de Serres is of opinion that 

 the habitat of Dreissena polymorpha (a kind of mussel 

 which abounds in many of our rivers and canals) was 

 originally marine, from the circumstance of the shells 

 being found in tertiary strata of marine formation. The 

 Russian traveller, Pallas, who first discovered or made 

 known this species, described one variety of it as marine 

 and the other as inhabiting fresh water. Many of the 

 marine Mollusca which live on the sea-shore (some of 

 them even beyond the reach of the tide) pass the greater 

 part of their time out of water ; and the same remark 

 applies to some freshwater snails, such as Limn^a 

 peregra and Ancylus fluviatilis, which are as often found 

 on dry land as in their natural element. Succinea putris 

 (a land-snail) appears to be almost amphibious. Many 

 genera of bivalve Mollusca contain certain species which 

 are marine and others which have a freshwater habitat. 

 Even Teredo, Pholas, and Area are in this category. The 

 smaller Crustaceans seem also to be very indifferent to the 



