V.] INTRODUCTION. 



of the marine Testacea (Saxicava arctica or rugosa)is said 

 to be almost " cosmopolite," being diffused over all seas 

 from Baffin's Bay to that which washes the shores of 

 Australia. It has also a wide range of habitat in the 

 same seas, extending from low- water mark down to a 

 depth of 100 fathoms and upwards. The distribution of 

 Terebratula caputserpentis is nearly as extensive in re- 

 spect of area and depth of water. This is spread not 

 only over all the European seas, but also (although under 

 other names, viz. septentrionalis and Japonica) over a 

 great part of the North and South Atlantic, Pacific, and 

 Indian Oceans. In both of these instances the variation 

 of form and sculpture is very considerable, being coinci- 

 dent with, and probably caused by, the extent of habitat. 

 Philine aperta is found in every part of the seas of 

 Europe, as well as in Simon's Bay at the Cape of Good 

 Hope, and in the Australian seas. Saxicava, Terebra- 

 tula, and Philine represent three different orders of Mol- 

 lusca ; and I have cited them, for that reason, as exam- 

 ples of the extent of what is termed "geographical distri- 

 bution." The limits within which some other kinds of 

 Mollusca occur are also very wide ; and the Gulf-stream 

 transports to great distances pelagic or floating kinds, 

 such as Hyalcea, lanthina, and Spirula. But, considering 

 all these to be exceptions, it may be assumed as a 

 general rule, that there is no specific conformity between 

 the marine products of the temperate and tropical 

 regions, especially between the Mollusca which inhabit 

 that part of the North Atlantic Ocean which confines 

 the coasts of Europe, on the one hand, and the rest of 

 the Atlantic Ocean, as well as the Pacific, Indian, and 

 great Southern Oceans, on the other hand. No authen- 

 ticated case has been recorded of any marine West 

 Indian species having been found living in the European 



