1918] Packard: Quantitative Analysis of Molluscan Fauna 319 



made on shipboard at the time of dredging (see Sumner et al., 1914, 

 pp. 1, 111). At a number of stations the bottom was found to be 

 composed of two or more types of materials. These have been classi- 

 fied according to predominance of one type over that of the others. 

 For instance, a bottom which might be characterized as a muddy sand 

 is herein designated as sand and mud. Since objects for support are 

 essential to some mollusks, groups one and seven are considered in 

 which shells comprise a conspicuous part of the bottom material. Of 

 course in such a case the presence of shell generally indicates that 

 conditions have long been favorable to molluscan life, therefore the 

 larger numbers in such a group are not necessarily entirely due to 

 the shell element in the composition of the bottom. The figures given 

 are derived from table 1, and represent the averages per haul within 

 the group under consideration. 



TABLE 8 



The Eelative Abundance of Species and Individuals tor the Different 

 Types of Bottoms 



O 



1. Pure mud 



2. Mud and sand 



3. Mud and shells 



4. Sand and mud 



5. Pure sand 



6. Sand and gravel 



7. Sand and shells 



In interpreting these figures due allowance must be made for the 

 fact that the different types of bottoms are not represented by equal 

 numbers of hauls. "When the number of living individuals is consid- 

 ered, it is seen that the greatest numbers were taken on bottoms char- 

 acterized as being composed of sand and shells ; while the second 

 largest numbers come from bottoms of mud and shells. The pelecy- 

 pods are represented by the larger number of species per haul from 

 bottoms characterized as sand and shells, mud and shells being the 

 next in importance as regards the number of species per unit area. 



A study of the molluscan associations peculiar to these different 

 types of bottoms shows several interesting relationships. The list of 

 species occurring upon various types of bottoms is given below, thft 

 asterisk indicating that the specimen was dredged alive. 



