262 B. H. GRAVE. 



manage to right themselves by one of three methods : Sometimes 

 without extending the foot, they open the valves and clap them 

 together. After one or several trials, the body turns over upon 

 the hinge line as a pivot. The mantle must have played a part 

 in this by expelling currents of water in a direction such as to 

 cause the body to turn over. 



At other times, a position on the right valve is gained by one 

 or more short swims, the method being continued until the body 

 comes to rest on the right side. They usually manage to alight 

 upon the right valve after a few trials, and then they become 



quiet. 



THE BYSSUS. 



When specimens are allowed to lie undisturbed upon the right 

 valve, they usually become attached by numerous strands of 

 strong byssal threads. A short time only is required for this to 

 take place. They frequently become firmly fixed in from two to 

 five minutes and the threads are sufficiently strong to support a 

 weight several times that of the body of the Pecten. The byssal 

 threads pass through the notch in the right valve directly to the 

 support below. They adhere, to some extent, to the shell where 

 they come into contact with it. 



So long as specimens are kept lying upon the left valve, they 

 cannot, or do not, attach themselves by the byssus. Since the 

 byssal gland lies at the base of the foot, it is possible that the 

 notch, in the shell opposite it, is a structural adaptation directly 

 correlated with the function of the byssus. At any rate, because 

 the byssal threads extend through the notch, in place of over the 

 edge of the shell, the pull has less tendency to tilt the body than 

 would be the case if no notch were present. 



In order for the byssus to become attached to the bottom, it 

 is not necessary for the valves of the shell to be opened, since the 

 attachment of the byssus is frequently accomplished while 

 they are closed. The notch in the shell is sufficiently large to 

 allow the extension of the foot to the support during the process 

 of attaching the byssus. It seems that Dr. Drew has observed 

 this process in individuals of Pecten irradians, to quote : 



" An individual of Pecten irradians placed in a glass dish of sea 

 water will sometimes protrude its foot from the shell, apply it 



