PHYLOGENY OF THE PELECYPODA. 347 



Smith figures a deep-sea species, which he calls Pecten aviculoides, on account of the 

 close resemblance which it bears to Avicula, although he considers it a Pecten. Again, 

 a fossil fi'om the Lias shales, as Professor Carpenter notes, was by one author named 

 Avicula and by another Pecten, the mixture of characters being such as to sanction its 

 being placed in either genus, according to the value attributed to different features. 

 Carpenter affirms that it is a true Pecten because of a coarse shell structure like Pecten 

 and an absence of prisms found in Avicula. Both these forms may properly be consid- 

 ered as representing in the adult a combination of types which I have shown is traceable 

 in early stages of the develoi)ing young. 



Whereas very young specimens of Pecten irradians ai'e yellowish-brown, older young 

 and adults are commonly bluish-gray, the coloration often being less or wanting in the 

 furrows of the upper, left valve. The lower valve may be of the same color as the up- 

 per, but is generally less pigmented, especially in young individuals, and specimens are 

 frequent in Avhich the upper valve being dark blue-gray, the lower is nearly or rpiite 

 white. This difference l)ears a direct relation to the amount of light to which the two 

 valves are exposed, tlie ])aler being the lower valve, which is constantly turned away 

 from the light. In studying other species of Pecten, many of which are highly colored, 

 we find that the same condition exists, the lower right valve lieing Avhite, or of a paler 

 color than the left upper valve, which may be brilliantly colored: I'ed, brown or yellow. 

 An example of a striking contrast is jhnuskcm (Pecten) jcqwnicum, in which the upper 

 valve is rich red, the lower, pure white. In the Spondyli, which are attached by the 

 right valve, the lower valve also is white, or paler than the upper left valve, which is 

 turned toward the light and is often highly colored. Fischer notes the difference in 

 color of the valves in Spondyli, and states that an abyssal species of Spondylus has both 

 valves blanched. This relation of the coloration to the amount of light received by the 

 two valves in bivalves is more or less noticeable in the lower and upper side of univalves 

 and is briefly considered by Tryon. Dr. Dall (16) considers the allied subject of the 

 coloration, surface marking and thinness of the shells of abyssal species, in his interest- 

 ing conclusions published in the Report of the Blake Mollusca. 



Pecten rnagellanicus Gmelin (P. tenuicostatus, Mighels). A number of young speci- 

 mens of this laj-ge species measuring from 2-6 mm. in height were dredged at several 

 stations by the U. S. Fish Commission Steamer Albatross. They ai-e entei'ed as No. G2307 

 in the Smithsonian Institution Catalogue. Sharply-defined but quite small prodisso- 

 conchs, like those figured in Pecten irradians, mark the umbos of all well-preserved spec- 

 imens. Fig. 38, p. 348 represents one of the smaller specimens. The early dissoconch 

 as indicated by the lines of growth is similar in form to Pecten irradians, PI. xxviii, 

 figs. 9-10, no ears having develoi^ed and there is a deep byssal notch in the right valve 

 although it is slightly indicated in the adult. The byssal notch of the right valve, how- 

 ever, bears no teeth as in the other species described. The hinge line equals nearly the 

 whole width of the shell as in Pecten irradians, PI. xxviir, fig. 12; but, as in that species, 

 differs in this character from the adult. Tlie hinge area on either side of the cartilage 

 pit bears a wide hinge plate, sinuous in outline and marked by numerous flutiugs. The 

 flutings are nearly perpendicidar (o the hinge line, but are slightly inclined toward the 

 umbos. Although cliaiacteristic of the young, this hinge plate is wanting in the adult. 



