PHYLOGENY OF THE FELECYPODA. 347 



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

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

 a fossil from 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 j^risms found in Avicula. Both these forms may pi-opei'ly be consid- 

 ered as repi'esenting in the adult a combination of types which I have shown is traceable 

 in early stages of the developing yoimg. 



Whereas very young specimens of Pecten irradians are yellowish-brown, older young 

 and adults are commonly bluish-grny, 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- 

 ])er, but is generally less pigmented, especially in young individuals, and specimens are 

 fi'equent in which the upper valve being dark blue-gray, the lower is nearly or quite 

 white. _ This difference bears a direct relation to the amount of light to which the two 

 valves are exposed, the paler 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 being white, or of a paler 

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

 An example of a striking contrast is Amusium (Pecten) japonicum, 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 magellanicus Gmelin (P. tenulcostatus, Mighels). A lunnbcr of young speci- 

 mens of this large species measuring from 2-6 mm. in height were dredged at sevei-al 

 stations by the U. S. Fish Commission Steamer Albatross. They are entered as No. 62307 

 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 rei)resents 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 developed 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. xxviu, fig. 12 ; but, as in that species, 

 difiers in this character from the adult. The hinge area on either side of the cartilage 

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

 flutings are neai'ly perpendicular to the hinge line, but are slightly inclined toward the 

 umbos. Although chai'acteristic of the young, this hinge plate is wanting in the adult. 



