METAZOA MOLLUSCA. 203 



Rhombopteria (PI. 385) represents a branch from 

 the primitive Nuculoid ancestral form, and is the probable 

 ancestor of the Aviculidae to which Pecten belongs. Its 

 shell was oblique, and it had a straight hinge line which 

 extended on either side of the umbos. 



The young of another genus, Pterinopecten, resembles 

 the adult Rhombopteria, while the hinge line of the adult 

 is long and the ears slightly developed. The young 

 Aviculopecten resembles the adult Pterinopecten but the 

 adult has a shortened hinge line and a much greater 

 development of the ears. 



The shell of the young Pecten (PI. 386, fig. i, viewed 

 from the left side; fig. 2, the same from the right side; 

 x 50 diameters) has the embryonic sheH or prodisso- 

 conch which represents the ancestral Nucula while the 

 succeeding stages resemble Rhombopteria, Pterinopecten, 

 and Aviculopecten. At first there are no plications and 

 the prodissoconch is without ears. According to Dall l 

 the very young valves of many species of Pecten have the 

 transverse groovings of the hinge, representing the teeth 

 of Nucula and Area. Fig. 3 is an older stage, x 40 

 diameters, and fig. 4 shows the fleshy animal at the same 

 stage. The two mantle borders are free and each posses- 

 ses a single row of eyes which alternate with single ten- 

 tacles. In a later stage two tentacles alternate with one 

 eye. The animal uses its long narrow foot actively so 

 that it is finely developed. Fig. 5 is the same shell viewed 

 from the right side, while fig. 6 is an older shell, x 16 

 diameters. The plications and ears are now well devel- 

 oped. The two borders of the mantle are extended to 

 form a tube just under the dorsal ear. Here the effete 

 matter is carried away in the outgoing current of water, 

 the direction of the current being indicated by an arrow 

 (fig. 6) . The gills of the very young Pecten are probably 

 four sets or two pairs of straight filaments, but when the 



1 Amer. Journ. Sci., ser. 3, XXXVIII, 1889, p. 459. 



