260 B. H. GRAVE. 



the currents of water expelled from the mantle chamber, it is 

 enabled to swim either forwards or backwards, although it usually 

 swims with the opening of the valves directed forwards. 



A perfectly symmetrical shell is the form best adapted to swim- 

 ming, and the presence of any irregularity in it, such as that just 

 mentioned in the pecten, is to be explained either as an adapta- 

 tion to habits other than swimming, or as a structure, inherited 

 from an ancestral form, and not as yet obliterated through adap- 

 tation to the swimming habit. 



Although adult specimens were kept in aquaria all summer, no 

 method of locomotion other than smimming was noted, and no 

 clue was gained as to habits which would in any way explain the 

 function of the notch in the right valve. They neither attached 

 themselves by a byssus, nor used the foot for locomotion. Young 

 specimens, however, showed much more activity than the adults, 

 and some observations on their habits are recorded in the follow- 

 ing pages. 



Concerning the function of the notch in a related species, Pecten 

 tenuicostatns, Dr. Drew writes as follows : " I have been unable 

 to satisfy myself as to the function performed by this notch. The 

 sense tentacles on the mantle margin, opposite the notch, are 

 somewhat longer than those adjacent, but I have been unable to 

 determine that they have a special function or that they are espe- 

 cially advantageously placed." 



THE SE^SE OF POSITION. 



The Pecten lies habitually upon the right valve and if placed 

 upon the left, immediately turns over. When lying upon the 

 left valve, it seems to feel the same sort of discomfort which a 

 frog, or other animal with well developed balancing organs, feels 

 when placed upon its back. However, after turning them over 

 repeatedly, they sometimes remained resting on the left valve for 

 several minutes. 



THE ASYMMETRY OF THE VALVES. 



When dropped through a considerable depth of water, Pectens 

 settle about as frequently upon the left valve as upon the right. 



: The University of Maine Studies, No. 6, September, 1906, p. 7. 



