1918] Packard: Quantitative Analysis of Molluscan Fauna 321 
Most of the species listed above occur in several groups of quite 
dissimilar character. This would suggest that the occurrence of a 
species at a certain locality does not give a true idea of its ecological 
relationships. The relative abundance of a species within a dredge 
haul gives a clue as to the optimum environment for that species, and 
therefore may well serve as the basis for studies in faunal associations. 
For this reason the average per haul for each species has been caleu- 
lated. The group in which the highest average falls would appear 
to represent that type of bottom best suited to the mollusk in question. 
Such a list is given below. The number of hauls is possibly too few 
to more than suggest the broad outlines of such molluscan associations. 
The following species are arranged according to their relative 
abundance on the different types of bottom: 
Group 1. Pure mud 
Living: None 
Dead: Gemma gemma var. purpura 
Mya arenaria 
Psephidia ovalis 
Columbella gausapata 
Odostomia franciscana 
Group 2. Mud and sand 
Living: Cardium corbis 
Gemma gemma var. purpura 
Crepidula nivea 
Dead: Pholas pacificus 
Group 3. Mud and shells 
Living: Macoma nasuta 
Modiolus, ef. rectus 
Zirfaea gabbi 
Dead: Cardium ecorbis 
Zirfaea gabbi 
Turbonilla franciscana 
Turris incisus? 
Group 4. Sand and mud 
Living: Psephidia ovalis 
Tellina buttoni 
Dead: Mytilus edulis 
Ostrea lurida 
Nassa mendica 
Group 5. Pure mud 
Living: Phacoides tenuisculptus 
Turbonilla franciseana 
Group 6. Sand and gravel 
Living: Tellina salmonea 
Dead: ' Hinnites giganteus 
Macoma inquinata 
Monia macroschisma 
Saxidomus nuttalli 
Tellina salmonea 
Turbonilla keepi 
