1142 The American Naturakst. _[December, 
From Leptodesma-like forms the Pinnid& are probably de- 
rived. This is considered a degradational group on account of 
the high development of the anterior adductor (a larval charac- 
ter), prismatic structure, and other peculiarities of some mem- 
bers. The Pterinea group is considered a side issue from the 
Rhombopteria stock. Pterinopecten is an important branch 
from the Rhombopteria stock. In the young it closely resembles 
Rhombopteria. It forms the base of the Pecten series. Aviculo- 
pecten is the next ascendant step in the Pecten series. In stages 
of growth it resembles, first, Rhombopteria; second, Pterinopec- 
ten. Pecten, the representative of the family, is the summit of 
the stock. As we have shown, the prodissoconch resembles the 
nuculoid radical. The first nepionic stage resembles Rhombop- 
teria; the second and third nepionic stages, resemble respec- 
tively Pterinopecten and Aviculopecten. Lyriopecten and Creni- 
pecten are considered side issues from Aviculopecten. From 
the Pecten stock important issues are given off. These are the 
Hinnites, Lima, Amusium, Anomia, and Dimya groups. 
The phylogenetic relations of the Aviculidz and allies are con- 
sidered in more or less detail in the last section of my paper, 
which is closed by a tabular view representing graphically the 
various genetic relations expressed. 
