INTRODUCTION. Panama" Shells. 9 



than an equal variety of stations. The number of species in 

 any region appears to be connected only with climate and 

 variety of stations. 



It is scarcely necessary to consider the question, whether 

 any of the shells on the opposite sides of tropical America 

 could have had a common origin. Although in some points 

 the Caribbean sea and the Pacific approach within 40 or 50 

 miles in a direct line, it. is difficult to imagine any adequate 

 means of the intercommunication of living marine mollusks. 

 If human agency had transplanted any species, we should 

 have expected that the edible species would have been selected. 

 The following are some of the edible Caribbean species, in the 

 order in which they have appeared to us in Jamaica to be 

 most commonly used : Trochus pica, Pyrula melongena, 

 Ostrea folium, Area Noe, (or an undescribed analogue of this 

 species), Strombus gigas, Turbo coelatus, Avicula crocata, 

 &c. At Panama, the edible species which we observed are 

 in the same order, Area tuberculosa, Venus discors, Ostrea, 

 two or three species, Area grandis, Murex radix, Pyrula pa- 

 tula, &c. All of these are remarkably distinct, if we except 

 the Pyrulae, which are analogues. If it should be supposed 

 that the analogues might have had a common origin, it will 

 be seen that the entire list of analogues comprises a full pro- 

 portion of the rare or minute species, which are not likely to 

 have been noticed by the inhabitants. 



Birds are commonly called in to aid the imaginary dis- 

 persion of species. On the Isthmus, the buzzards are best 

 able, with their extraordinary powers of flight, to transport 

 them ; but their habits of feeding are wholly at variance 

 with such a supposition. The water birds which feed on the 

 marine mollusks are unknown to us. But if their agency has 

 been effective, it is remarkable that we can identify only one 

 species, and that doubtfully, (Crepidula vinguiformis), on both 

 sides of the Isthmus, and that the habits of this mollusk, at- 



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