59 



Concerning Shore and Bottom Conditions. — Investigations show 

 that bays and other partially inclosed bodies of sea water maintain a 

 different aggregation of life-forms from that found out on the open 

 coast, where the waves are more vigorous and life more strenuous. 

 The group of animals on sandy bottom is markedly different from 

 that on muddy bottom ; so also do rocky and shelly bottoms differ 

 from each other in the kind of animals they maintain and from the 

 other kinds of bottom mentioned. The same holds good for the 

 shores ; they differ from one another and from the various kinds of 

 bottom in the species preferring them. Of course, some species ap- 

 pear to thrive equally well on two or more kinds of bottom or shore ; 

 but, on the other hand, each kind of bottom or shore has its own pecu- 

 liar characteristic species. In general, it may be asserted with some 

 degree of assurance that all the species of any one genus, if there 

 is nothing known to the contrary, prefer similar shore or bottom con- 

 ditions. For example, if Scapharca transversa is always found pre- 

 ferring sandy shores, it is not far wrong to say that other species 

 of Scapharca very like also prefer sandy shores. In the tables, how- 

 ever, this supposition has not been employed, and any word used in 

 describing the kind of shore or bottom inhabited by any species is 

 given on the authority of such workers as Verrill, Dall, Gould, Say, 

 Binney, Fisher, Walther and Tryon. In discussing this phase of the 

 subject, however, it does not seem amiss to employ the supposition 

 just mentioned. Again, the kind of material upon which the marl 

 beds rest and which is mixed in with the shells as they are exposed to- 

 day, where there is evidence to suppose that these shells have not 

 been carried by currents and deposited as they are found, but occupy 

 the same position in the main today that they occupied when they 

 were living in Pleistocene times — then, let it be said, this material 

 must be taken into account and given serious weight in drawing con- 

 clusions concerning the shore and bottom of the Pleistocene seas and 

 bays. 



From the tables it seems that about thirty species of the entire 

 number are found living in mud. Of this number, nineteen prefer 

 the mud to any other kind of bottom, and thirteen seem to be found 

 only on muddy bottom, and six are found in the mud of estuaries or 

 other places of brackish water. More than half the mud-loving 

 species are the only species of their respective genera, and so can 

 be of little weight in attributing this characteristic to others of their 

 genera. Forty-six species are found to live on sandy or gravelly 

 bottom or shore, and of these, twenty-nine prefer sand above all 



