GEOGRAPHICAL DISTRIBUTION OF THE MOLLUSCA. 155 



These "natural barriers," as they were called by Butfon, retard 

 or altogether prevent the migrations of species in particular 

 directions. 



I am about to speak in detail of geographical provinces ; but 

 as I do not regard them from the saijie point of view as some of 

 my predecessors, I have deemed it inadvisable to occupy as 

 much space in the discussion as Woodward, for instance, has 

 done. In fact, except where provinces are ph3^sically circum- 

 scribed, as an isolated island, for instance, their boundaries are 

 more artificial than natural, and might, in many cases, be greatly 

 changed without depriving them of that measure of exclusive- 

 ness which is held to constitute a distinct fauna. As to the 

 distribution of the species, if we take a hundred species of the 

 Panamic Province or Region, for instance, we shall find that we 

 have a hundred areas of distribution, and no determinate specific 

 centre. If, instead of the present divisions of Aleutian, 

 Californian, Panamic and Peruvian Provinces, we should locate 

 provinces of equal geographical extent anyiohere on the coast, 

 we should find as many species peculiar to them as to the 

 former. 



Grranted that geographical provinces or regions do not fulfill 

 the ideas which they were originally intended to convey, they 

 are still useful (as many other partly artificial systems are 

 useful), in recalling the character of the fauna of a given 

 section of the world by means of a name. To subserve this 

 purpose, it is advisable that the names and boundaries should 

 not be subject to change, just as it is advisable not to change 

 the name of a genus or species ; I therefore, present the various 

 provinces as they have been generally accepted, without reference 

 to their varying values. Pelagic mollusks (cephalopods, 

 pteropods, heteropods), have the most extended distribution, 

 and, of course, cannot be considered properl}^ as belonging to 

 any particular faunal regions. The same may be remarked of 

 deep-sea species, as the conditions under which the^^ exist are 

 altogether difl'erent from those which determine the distribution 

 of littoral species. The latter only are taken into consideration 

 in forming marine provinces. 



Amongst the genera of marine shells there are some which 

 have been considered particularly indicative of climate. From 

 the Arctic list the following may be taken as examples of the 

 shells of high latitudes ; those marked * being fou.nd in the 

 southern as well as in the northern hemisphere : — 



Buccinum, *Trichotropis, *Rhynchonella, *Astarte, 



*Chrysodomus, Yelutina, *Crenella Cyprina, 



*Trophon, Lacuna, Yoldia, Glycinieris. 



Admete, * Margarita, 



