DAVENPORT. — EVOLUTION OF PECTEN. 137 



5. Color Variations and Abnormalities. 



The three collections differ in general appearance as much as in dimen- 

 sions ; that is, they have certain features in common and others that are 

 peculiar. The upper valves from all localities are more highly colored 

 than the lower valves. This condition is found in pectens in general 

 and is probably due to the greater exposure of the upper valves to the 

 light. The lower valves, however, sometimes exhibit concentric rings of 

 color. Pecten opercularis from the three localities tends to have on 

 the upper valve a median light-colored rib and a submarginal one on 

 each side. Concentric rings of color are common, especially in the Irish 

 Sea and Firth of Forth shells. Antero-posterior stripes of white are 

 frequently present in the young shells from all localities, but especially 

 in those from the Forth. 



The upper valves from Eddystone are the lightest in color of all in the 

 collection, probably due to their living at a greater depth, where less sun 

 li<dit penetrates. Relatively few barnacles, but many serpula tubes, ad- 

 here to them. The Firth of Forth valves are very muddy, indicating 

 that there was some mud on the " shelly " bottom. They are densely 

 covered with barnacles and bear a few serpulas. They vary in color 

 from white to dark red-brown. The Irish Sea valves are, on the whole, 

 the darkest of all, being nearly uniformly red-brown. Of the variations 

 in ground color, yellow is the commonest, being found in all localities. 

 From the Irish Sea I have noticed also orange, pink, salmon, and lav- 

 ender. The color variation from yellow to red is that characteristic of 

 lipochromes, whose presence has been detected in the shells of Pecten* 



The abnormalities which I have come across may be classified as follows : 

 (a) Interference with the development of the shell by organisms growing on 

 it; and (b) abnormal rays, due to abnormal crenulations of the mantle. 



a. Interference with Development by Growth of Organisms on the Shell. 

 — The effect was repeatedly noticed in valves from Eddystone, and only 

 from here. Numerous Serpulas grew on the valves, and as they readied 

 the free margiu of the shell turned toward the opposite valve. Thus a 

 barrier was interposed at the margin to the further enlargement of the 

 shell, which is, consequently, notched at this point. That this peculiar 

 modification of the shell was found in this locality only, is correlated with 

 the fact that there are more Serpulce on the shells here than elsewhere. 

 In a shell from the Firth of Forth the growing edge has been interrupted 

 by a mat of escharine Bryozoa.\ 



* Teste, Newbigin (18'J8), 189. t No. 15775, University of Chicago collection. 



