PIIYLOGENY OF THE PELECYPODA. 333 



succeeded in getting' very young specimens of either genus. The observations which 

 lun e been made on the development of these genera are given in section xvi. 



IX. Pecten. Anatomy and Habits. 



Material collected at Buzzards Bay, in 1838 afforded an opportunity to study Pecten 

 irradlans. Lam., fig. 37, which is common on the southern shores of Cape Cod. Through 

 the kindness of Dr. Dall, I have also l)een able to study the young of several other species 

 from the collections of the Smithsonian Institution at Washington. The consideration 

 of the anatomy and life habits of Pecten irradians is taken up in this section and a study 

 of the shell of that and the other species studied is described in section x. 



On the fourth of August, in examiningthe glass plates inmy draiu-pipe traps (described 

 in section ii), a number of extremely young Pectens were found. These specimens 

 proved of great interest on account of the shell form 

 and sti'ucture and also on account of anatomical feat- 

 ures and the habits which I had the opportunity of 

 obscr\'ing, as they were kept alive for several days. 

 No ver}' young specimens were found on the sand- 

 bar immediately around the drain pipes although 

 diligently searched for, and the traps were a most 

 invaluable assistance in catching extremely young 

 molluscs which were easily seen on ])1ates of glass by 

 allowing- the light to strike them obliquely. 



The si^eeimens found on the fourth of August 

 varied slightly in size as shown in PI. xxviir, figs. 

 9-12. A few of the same age were found on the glass 

 the next day; but later none of these youngest stages Fig. 37.— Achut I'm-un in-adians, Lam. Life 

 were seen. The habits of these were alike in all ^''■'- (From Binneys Gould.) 

 the individuals observed and the anatomical features were the same as far as traced. 



The young Pectens, PI. xxvm, figs. 1, 9, 12, were free, crawling on the glass. No 

 byssus was observed until a considerably later stage and it is safe to say that this early 

 period has not yet developed the habit of byssal fixation. It is a free period, preceding 

 the byssal period. The young scallops were extremely active crawlers with a highly de- 

 veloj^ed foot, which is ca])able of extreme prolongation, PI. xxvnr, fig. 1, and is often 

 produced beyond the edges of the valves to a length equal to the whole height of the 

 shell. While lying on the right valve, the foot is extended through the notch close un- 

 der the hinge line and is attached by a sucker-like action to the glass, shortening its 

 length. It thus drags the shell along on its right side. The motion is very rapid and 

 the little creature quickly passes over a considerable distance. Shells were frequently 

 observed to clap as in the adult; but they did not SAvim by this means and it is quite pos- 

 sible that they were not capable of doing so, as special features of the mantle which 

 serve the adult in swimming were not yet developed. The foot is so mobile and exten- 

 sible that it may be twisted and turned in any direction at the will of the animal. 



Older scallops and the adidts lie on the right valve, and are rarely found lying on 



MEMOIRS BOSTON SOC. NAT. HIST., VOL IV. 45 



