124 A COMPARISON OF SOME PEGTENS FROM THE EAST 



III. CLOSE RELATIONSHIP OF THE TAMPA AND SAN DIEGO SHELLS. 



Dall ('98, p. 711) calls P. ventricosus the Pacific coast analogue of P. dislocatus 

 Say. I cannot doubt that there is more than an analogy between the species. The 

 grounds that lead me to conclude that there is a close relationship are as follows : 



i. General Aspect of the Shells of the Two Lots The maximum size is about the 



same for Tampa and San Diego shells, 65 millimetres and 62 millimetres respectively. 

 In the two lots studied by me, however, the mode of the Tampa shells was 54 milli- 

 metres and the mode of the San Diego shells was 40 millimetres, so that the San 

 Diego shells run smaller than those of the Gulf, which is possibly due to the fact 

 that they were collected at a different time of the year, i.e., were younger. The 

 weight and symmetry of the valves, the form of the rays, and the general color are 

 also similar, although the lower valves of the Tampa shells are whiter and the upper 

 valve more strikingly variegated than is the case with the San Diego shells. 



2. Color Markings — The color markings show resemblances in detail (PI. IX, 

 Fig. 1). In both lots the right (lower) valve may be without markings, though the 

 San Diego form never has the pearly-white color of the Tampa form. Thus the color 

 formula* of the unmarked Tampa lower valves is W. 87, N. 13; of the San Diego 

 valves, W. 45, N. 20, Y. 23, R. 12. Likewise the right valves from both localities 

 show a mottling of brown to blue-black on a uniform ground. In extreme cases 

 the entire shell has a dirty red-orange ground color which, near the beak, is expressed 

 in the case of the Tampa species by W. 13, N. 18, Y. 12, 0. 34, R. 23, but in the 

 San Diego species is less orange and red, as the following color formula shows : W. 25, 

 N. 22, Y. 18, 0. 20, R. 15. The ground color of the upper valve varies in the Tampa 

 lot from an ebony black to a very dirty orange red: W. 5, N. 41, Y. 10, O. 22, R. 22. 

 In the San Diego form there is, in the darkest valves, 10% to 20% of white, and the 

 valves with the greatest amount of orange have the formula: W. 22, N. 29, Y. 27» 

 O. 22. Thus in both valves of the San Diego shells yellow seems to replace the red 

 of the Tampa shells. ! 



The upper valve of the Tampa Pectens has in the midst of the dark ground certain 

 light-colored rays, henceforth called the "white" rays, although by no means 

 always white. The most constant of these is a nearly middle rib. This white ray 

 usually lies nearer the posterior than the anterior end of the series; in only about 2% 



* The formulae represent the proportions, per cent, for the standard colors in the Maxwell color mixer made 

 by the Milton Bradley Company. The letters are the initials of the primary colors; also, W. = white; N. = black 

 (nigrum) . 



