THE EUROPEAN SEAS. 



231 



brittle ; tlie animals were not buried in the sand, 

 but inhabited the Conferva crassa, in which the 

 majority of the specimens were found creeping 

 about." 



In warmer regions, as in the South Lusitanian 

 province, our brackish Gasteropods are replaced 

 by species of Cerithia, Melania, and Ampullaria. 

 Corbula, though a deep-sea form at times, follows 

 the habit of Mya, and in warmer regions makes its 

 appearance in brackish waters. A gradation of 

 form may be traced from typical Corbulce to JPoto- 

 momya, according to the medium in which the 

 forms occur. This is an interesting fact, because 

 true Corbulce will be found at the interchange be- 

 tween salt and fresh water conditions as far back as 

 the commencement of the cretaceous series, and 

 again, in the fluvio-marine beds of the tertiary 

 series of the Isle of "Wight, where Potomomya also 

 occurs in most wonderful profusion. 



That like conditions produce like assemblages 

 may be seen by comparing the Black Sea fauna 

 (p. 201) with that of the Baltic (p. 88). 



In the Gulf of Bothnia many of our common 

 English air-breathing pond-snails have habituated 

 themselves to the slightly-saline waters of that part 

 of the Baltic ; such is also the case in the Sea of 

 Azof. The changes produced by the degrees of 

 saltness of the water on certain fresh-water . forms 

 have been noticed by Ed. Forbes in his obser- 

 vations on the coast of Asia Minor. In this re- 



