Collection and Preparation 19 



moved should be replaced in its original position; do not expose to 

 drying and needless death the many small creatures that have been 

 disturbed by the search. 



Beyond the limits of the beaches search for living mollusks 

 must be based upon some knowledge of their habits and stations — 

 stations having reference to the particular conditions of environ- 

 ment which are congenial to various species. Association of certain 

 species of mollusks with definite characters of environment is almost 

 invariable. Thus species of Cerithidea, Cerithium, Marginella, Melon- 

 gena, Anomalocardia, Polymesoda, and several small Tellinas coin- 

 habit tidal salt flats and the intertidal zone of inside waters. Littor- 

 ina, Melampus, and Truncatella live about salt marshes, often out of 

 water for long periods. Ostrea and Modiolus live about the mangrove 

 fringes of the flats and bayous and are out of water at extreme low 

 tide. Barnea truncata is likely to be found in black mud near man- 

 groves, while Cyrtopleura costata lives in deeper water, usually where 

 there is an admixture of gravelly bottom with a substratum of marl 

 or silt. 



On grassy bottom are found Modulus, Pyrene, Cerithiwm, Bit- 

 tium, Rissoina, Marginella, Smaragdia, and at certain seasons, Pecten. 

 On sandy bottoms, living buried beneath the surface with siphons 

 extended upward into the water are Chione, Mercenaria, Transenella, 

 Pitar, Macrocallista, Lucina, Tellina, Mactra, Tagelus; and preying 

 largely on these bivalves Natica, Polinices, Terebra, Conus, Nas- 

 sarius, Cantharus, and Urosalpinx move freely about. Wherever 

 Mercenaria campechiensis is found there also are Busycon and Fas- 

 ciolaria. Eupleura is often associated with a bottom weed known 

 commonly as "rolling moss". 



Dentalium, Corbida, Nucula, Nxccidana, Lyonsia, and Cuspidaria 

 are found in soft, oozy bottoms beyond low tide mark, although 

 Lyonsia is also found in shallow water. On outcroppings of hardpan 

 and about reefs associated with corals, corallines and sponges are 

 found Murex, Cancellaria, Leucozonia, Xenophora, Trivia, and 

 Scaphella junonia, with Cha'rna, Pseudochama, Echinochama, Spon- 

 dylus, Ostrea- cristata, and 0. frons. Pecten gibbus is a Gulf species 

 apparently living in large, scattered and migratory colonies 



