T. Bland on the Geographical Distribution of Mollusca. 399 



found, (as I am informed by Mr. Cnming,) in the central islands 

 of the Pacific. On the other hand, there are Galapageian species 

 of Oniscia and Styhfer, genera common to the West Indies and 

 to the Chinese and Indian seas, bnt not found either on the west 

 coast of America, or in the central Pacific. I may here add, that 

 after the comparison by Messrs. Cuming and Hinds of about 2,000 

 shells from the eastern and western coasts of America, only one 

 single shell was found in common, viz., the Purpura patula, 

 which inhabits the West Indies, the coast of Panama, and the 

 Galapagos. We have, therefore, in this quarter of the world, 

 three great conchological sea-provinces, quite distinct, though 

 surprisingly near each other, being separated by long north and 

 south spaces either of land or of open sea." — p. 162. 



Professor Edward Forbes in his able paper, "0?i the connection 

 between the distribution of the existing Fauna and Flora of the 

 British Isles, and the Geological changes which have affected 

 their area, especially daring the epoch of the Northern Drift,''^ 

 published in the first volume of the '• Memoirs of the Geological 

 Survey of Great Britain, ^^ {London, 1846,) gives a sketch of the 

 distribution of marine Mollusca on the British shores, chiefly the 

 results of his own observations. Prefatory to this sketch, he 

 observes, — 



" Our knowledge of the species and distribution of the British 

 Mollusca is very complete, and sufficient to enable us to apply it 

 to the elucidation of geological problems with safety and efl"ect. 

 When we consider the perfect state in which the testaceous spe- 

 cies are preserved, and the facility of specific identification afforded 

 by their shells, this becomes of great importance. In all ques- 

 tions respecting the age of sedimentary strata, the evidence af- 

 forded by the fossilized remains of the MoUusks must, from its 

 completeness, always take precedence of that derived from any 

 other class of animals. Though our native existing species have 

 been well determined, there is no one work upon them, to which 

 the geologist can be referred with safety, nor any comprehensive 

 essay as yet published on their distribution." 



Having cursorily traced the progress which has been made in 

 the study of the geographical distribution of shells, (so far as a 

 reference to a limited library would permit,) with the particular 

 view of shewing its value, and at the same time the incomplete- 

 ness of our knowledge with regard to any one zoological province, 

 we would point out the causes which have given rise to many 

 errors of fact and opinion on the subject, and seriously tended to 

 impede its advancement. 



These causes unquestionably are, to use the language of Dr. 

 Gould, [Introduction to the Mollusca of the Expedition Shells,) 

 that "reliable notes of localities have not been taken," and that 

 shells " are regarded as specifically identical, which, on careful 

 comparison, are found not to be so." To these, we think, a third 



