200 LECTURES ON 



The most common Calyptrasid on the backs of our Spondylus valves, 

 however, was Crepidula aculeata, Gmel. It was first described from 

 West Indian specimens, which are generally dead and worn, in col- 

 lections, and afterwards re-described from fine West Coast shells as 

 C. hystrix and C. echinus, Brod. The stunted Northern form was 

 named C. Californica by Nuttall. The rule is laid down by some 

 American authors of great celebrity that no species can be common 

 to the Atlantic and Pacific waters. Accordingly, when the same 

 form reappears on the wrong shore, it is their custom to re-describe 

 it, there being always differences by which a few individuals can be 

 separated from each other. But it is well known by those who have 

 examined extensive series from different localities that each locality 

 may present the same species under very different aspects. A large 

 number of British shells live also in the Mediterranean, but in a 

 mixed collection it is generally easy to pick out northern specimens 

 from their southern congeners. So again the Panama shells (of iden- 

 tical species) can generally be separated from the Mazatlan; and these 

 again from those of Acapulco and Cape St. Lucas. Now if the east 

 and west coast shells do not differ more than those of Panama and 

 Mazatlan; nay, do not differ so much as those of either place among 

 themselves; it appears an argumetitum ad ignorantiam to describe 

 them as distinct species^ merely because ive cannot tell how they have 

 become distributed. On comparing Dr. Gould's descriptions of Pur- 

 pura pansa (Pacific) and P. patula, (West Indian,) with my own well 

 authenticated specimens, it appeared to me that the diagnosis of patula 

 Avas exactly fitted to the Mazatlan shells, while that of pansa belonged 

 rather to the shells collected by my brother at St. Vincent's. Our 

 knowledge of the fauna of each region is as yet too meagre to speak 

 on doubtful matters with any dogmatism, but the researches of modern 

 geology have already determined the fact that in the tertiary (Miocene) 

 epoch there was a communication between the two oceans; that very 

 remarkable Pacific shell, Malea ringens, having been found fossil on 

 the Atlantic coast. This interesting solution of a doubtful problem 

 is due to the research of Dr. Newberry, and is an instructive example 

 of the light which different branches of study throw upon each 

 other. 



We may now be allowed to predicate that old species, which have 

 survived since the Miocene epoch, may be expected to appear on both 

 sides of the peninsula; while those of modern creation may be ex- 

 pected to be distinct. Furthermore, the old species may be expected 

 to have more power of living under varied influences, and, therefore, 

 to be more variable in shape, and more widely diffused than those 

 more constant and local in their characters. The history of British 

 shells, which are more thoroughly known than those of any other 

 district in the world, furnishes many instructive instances of these 

 facts. In Mr. Searles Wood's work on the Crag Mollusca* the newer 

 tertiaries are divided into the Coralline crag, the Red crag, and the 

 Mammaliferous crag, (answering perhaps to the Miocene, Pleiocene, 



* Published in three parts by the British Pala^.ontographical Society, of which a copy 

 is in the Smithsonian Library. 



