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 ease the presence of shell generally indicates that 
conditions have long been favorable to mollusean 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 
Tur RELATIVE ABUNDANCE OF SPECIES AND INDIVIDUALS FOR THE DIFFERENT 
TYPES OF BoTToMs 
ES. re ae z 
“ a a ae Ae 8 « 
& re Sie ee ae RSS AGS 
o8 Be 8 Soe fo eo 3h  s8385 
BE Goethee aes eH fee . fee 
#8 oa 55 FR 2 FR FES 
1. Pure mud 11 3.2 3.2 13.9 2.6 6 
2. Mud and sand 14 4.0 3.7 41.8 3.6 A 
3. Mud and shells 4 7.0 8.7 82.5 5.5 1.5 
4, Sand and mud 5 9.8 8.8 33.8 7.8 2.0 
5. Pure sand 1 5.0 5.0 28.0 4.0 1.0 
6. Sand and gravel 4 8.25 7.5 2.0 7.0 1.2 
7. Sand and shells 4 11.5 10.0 174.0 9.2 2.7 
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, the 
asterisk indicating that the specimen was dredged alive. 
