PLATE 17 



Fig. 1. Comparison of extreme variates of each of the series figured in plate 

 5; Goat Island (left), Dumbarton (center), and Crockett (right). X 5. 



Pig. 2. Details of denticulation of a portion of the anterior median area of 

 a shell from Goat Island. X 75. 



Fig. 3. The same, from Dumbarton. 



Fig. 4. The same, from Crockett. 



Fig. 5. Shells from stagnant aquarium, discolored black by action of sulphur 

 bacteria. X 4. 



Fig. 6. Head of Teredo, with body and shell colored blue as a result of work- 

 ing in redwood in vicinity of an iron nail. X 4. 



[32S] 



