1138 The American Naturalist. [December, 
like the same stage of Perna (Fig 8) and Avicula (Fig. 9), and is 
traceable in origin to the same ancestral type, Rhombopteria 
(Fig. 16). The right valve of this stage of Pecten possesses a 
well-defined layer of prismatic tissue, though it is early lost. 
FIGS. 11, 12.—Young Pecten irradians, viewed from the leftand right sides ; 2, prodis- 
soconch. X about 37 diam. 
This is a good example of accelerated development, for prismatic 
tissue is characteristic of the adults of the Aviculidze, from which 
group the Pectinide were derived. 
In a later stage of growth of Pecten (Fig. 13) we find im- 
A NS 
> 
RD 
DSS 
S 
IE 
Tae Fae t 
munia N 
N N 
TI] 
' 
IG. 14.—Pecten irradians, still older ; 
showing foot, tentacles, eyes, and excur- 
rent osiphon. X r2 diam 
FIG. 13.—Young Pecten irradians; p, 
prodissoconch. X 3o diam. ; 
portant changes in form, which mark the beginning of features 
characteristic of the family. The hinge line is longer and termin- 
ated by slightly developed ears, plications originate in both valves, 
and the byssal notch is strongly marked. This stage is referable 
