MOLLUSCA. <i 



naturally provokes comparison with those of vegetation in alpine situations, where 

 atmospheric pressure is diminished. And it will be found that the influence is 

 much more conspicuous among the former ; though in both the most decided 

 difference will be traced to species rather than to groups. 



The character of the floor of the ocean is an important guide as to the animals 

 which may be expected to inhabit it. Oliva, Terebra, Voluta, Pecten and Donax 

 prefer a surface more or less sandy ; Conchiferce, generally, are prone to muddy 

 situations, and in such they are proportionately in numbers. Venus gnidia 

 occurs in muddy situations along the extent of the west coast of America 

 between the Bay of Guayaquil and San Bias, or about fourteen hundred 

 miles ; but it is not found in the harbour of Acapulco, the sandy floor of which 

 is very prolific in shells, where it is replaced by Venus cancellata, which is 

 partial to sand. And as lichens are scarce in a country which has little or no 

 forest, so, for an analogous reason, species of Fissurella and Chiton are only to be 

 expected on a gravelly or stony surface. The floor of the Bay of Guayaquil 

 consists of sand, mud, and all the intermediate states ; I have mentioned how 

 prolific some parts proved to us, yet in other and sandy situations the dredge was 

 used for hours without the smallest success. Wherever coral prevails its influence 

 is very manifest. Throughout the Pacific Ocean this is witnessed on a large scale ; 

 but it is more interesting to observe it on the western shores of America, where, 

 throughout its vast extent, small patches of coral only rarely occur. At the island 

 of Cano, near the Gulf of Nicoya, we dredged among coral, and found that the 

 shells all displayed a delicacy in their conformation, and particularly in their 

 colouring. There is no disposition to increased richness in colours, but the 

 reverse, some being nearly blanched, and others with tints more pale and delicate 

 than usual. In the Pacific, which is chiefly inhabited by shells that have 

 migrated from the Indian seas, its influence is extended to the size, and if 

 examined from west to east, many species will be found gradually to diminish. 



During our sojournings, we were frequently called on to notice the absence of 

 groups, which, from the climate and latitude, it might naturally be expected would 

 have been represented. It is certainly very curious that the two large genera of 

 Conus and Cyprcea have no representatives on the west coast of America south of 

 Guayaquil. In the Pacific, a great many large and important groups are entirely 

 absent, or very nearly so, particularly those which are abysso-pelagic. In this 

 ocean, the bottom is usually at an unfathomable depth ; and the means of search, 

 and most probably of habitation, are extremely limited. Its peculiar species are 

 few, being chiefly fed by migration from other seas ; thus there are no, or scarcely 

 any, representatives of Cancellaria, Pleurotomacece , Marginella, Nucula or Corbula. 

 The species met with are usually found very numerous in individuals, which I do 



