DAVENPORT. — EVOLUTION OF PECTEN. 125 



sea-water in this part of the Channel is 3.5 to 3.55 per cent, and the 

 mean snrface July temperature is about 62° F. (16.7° C.)* 



4. The fourth collection was obtained from the Bay of Naples in 

 30 metres, off the Posilipo. These were of small size, probably not 

 fully grown, and are not included in the present study. f 



Before attempting to compare the shells from different localities, it is 

 essential to consider whether they are comparable. Age is an important 

 factor of variation that must not be neglected. Over 99 per cent of the 

 shells measured had a dorso-ventral diameter between the limits of 40 and 

 75 mm. As they were all collected in the autumn, at which time the 

 new brood is very small, they were clearly at least one year old. From 

 analogy with Pecten irradians they do not live to be three years old. 

 Consequently they must be either one or two years old. Wliile the 

 pectens of the new brood may increase in diameter as much as a milli- 

 metre a day, the growth after the first year is exceedingly slow, aver- 

 aging from 5 to 10 mm. for the entire second year and much less 

 in the third year. Consequently the range in diameter (as given in 

 Table I) can be attributed in only small part to differences in age. The 

 material was all taken by dredging, and consequently quite at random. 

 There is nearly perfect correlation between the right and the left valves 

 in the dorso-ventral and the antero-posterior diameters as well as in the 

 number of rays. So that, for most studies, one valve alone need be 

 considered. The lower valve has usually been chosen. 



The geographical range of Pecten opercularis is, according to Locard 

 (1898), as follows. In the east Atlantic, to the north (at a depth of 

 between 9 and 187 metres), as far as the Lofoten Islands off the coast 

 of Norway. It is found on the coasts of Great Britain and Ireland, 

 France, Spain, Portugal, Morocco, and south to the Azores, Madeira, 

 and Canary Islands. To the west it has been dredged (by the " Light- 



* These shells were obtained for me in September and October, 1902, through 

 the generous assistance of tlie Marine Biological Association's laboratory at 

 Plymouth. I take this opportunity to express my obligation to Director E. J. 

 Allen, D.Sc, to Mr. R. A. Todd, who was in charge at the time of our visit and 

 advanced our work in every way, and to the other members of the laboratory 

 staflF. 



t To the Director, Professor Dohrn, Professor Eisig, Dr. LoBianco, and the other 

 members of that Mecca of zoologists, I am glad to return thanks for the oppor- 

 tunity to study at the Laboratory and for assistance received tliere. 



I gladly acknowledge, moreover, my indebtedness to the Smithsonian Institution, 

 under the direction of Professor Langley, for an appointment to the table of the 

 Institution at the Naples laboratory. 



