396 T. Bland on the Geographical Distribution of Mollusca. 



California, nearly in 28° N. lat. ; but none of them inhabit San 

 Diego, which is near 33° N. lat. In the same manner the south- 

 ern limit is found near the boundary between Peru and Chili, 

 between 22° and 24° S. lat. 



The reason why the range of the species south of the equator 

 is several degrees less than on the north side, is obvious in the 

 Antarctic current, which sets along the west coast of South Amer- 

 ica. In like manner, on the eastern coast of North America, a 

 polar current appears to extend the Arctic Fauna of marine shells 

 to 41° N. lat., and the cold current along the shores of the Middle 

 and Southern States seems to limit the tropical Fauna to about 

 26° or 28° N. lat. on the coast, although on the east side of the 

 Gulf Stream it extends to the Bermuda Islands in latitude 32° N. 



It will also be seen in the following pages, that a large number 

 of the species which occur at Panama were collected by Mr. 

 Cuming at the Gallapago Islands. But if any of the species oc- 

 cur in the Polynesian Islands, the number does not exceed three 

 or four, and in these cases the specific identity of the shells is very 

 doubtful. The western boundary of the Panama province may 

 therefore be made to include only the Gallapago Islands." — p. 5. 



Professor Adams gives a list of 20 species collected by him at 

 Panama and Taboga, which occur also at the Gallapago Islands, 

 from which it appears, that a larger proportion of the littoral than 

 of the pelagic known species are common to the Gallapago Islands, 

 and the main land ; and of the pelagic species, all which are 

 known inhabit moderate depths. 



With regard to the limits of the Caribbean province, Professor 

 Adams remarks that very few of the Caribbean species "occur 

 north of the Bahamas, or south of Brazil, although from various 

 sources of error many of them have been reputed to inhabit Eng- 

 land, and various other parts of the world. Although there are 

 several analogous species in the two provinces, in general there 

 is a great dissimilarity." 



The author thus concludes his Introduction : — 



" A great amount of valuable materials, for interesting generali- 

 zations on the number and distribution of the species of marine 

 Mollusks, has been collected by public and private expeditions. 

 But when we consider the immense profusion of species and of 

 individuals in tropical seas, and the confusion of the habitats of 

 many as exhibited in the present literature of conchology, so that 

 the plan of distribution is misrepresented, — that which has been 

 done only stimulates the curiosity to know more, and to know 

 more accurately. An expedition of circumnavigation^ with the 

 specific object of investigating the distribution of the testaceous 

 Mollusks, through several tropical marine provinces, would ac- 

 complish more than the discovery of many new species : it would 

 determine the distribution of a much greater number, in a man- 

 ner which would justly inspire confidence. 



