1132 The American Naturalist. . [ December, 
STUDIES OF PELECYPODA. 
BY ROBERT TRACY JACKSON. 
1 HIS paper is a brief summary of some of the more important 
and interesting facts and conclusions contained in a paper 
recently published by the author! The leading principles em- 
ployed in the paper are two: I. Professors Cope’s and Hyatt's 
theory of the acceleration of development, with its corollary 
that in the young, stages are found, the equivalents of which are 
to be sought in the adults of ancestral groups. II. That in the 
mechanical conditions of envirofiment and a study of the life- 
habits of animals, facts may be gathered which will throw light on 
the origin and meaning of external and internal anatomical fea- 
tures? In accordance with the above principles, young and 
adults, living and fossil species of many allied genera were taken 
up as one common study. 
The completed embryonic shell of Pelecypods differs from 
and is commonly sharply marked off from the succeeding shell 
growth. This embryonic shell is compared to the protoconch of 
cephalous molluscs, and as it is bivalved it is termed a prodisso- 
conch. Carrying out the same terminology, the succeeding shell 
growth is termed a dissoconch. In my paper a prodissoconch is 
described, or at least mentioned, as existing in thirty-nine genera 
of Pelecypods. 
Prof. Hyatt's classification of stages of growth and decline* is 
considered at length in its application to the Mollusca. Some 
alterations in that valuable classification were deemed necessary, 
and are introduced. R 
1 Phylogeny of the Pelecypoda, the Aviculidze, and Their Allies. Mem. Bost. Soc. 
Nat. Ser Vol. IV., No. 8, July, 1890, pp. 277-400, Pls. XXIII.-XXX. 
n a p&per soon to be oec in this journal, I consider some cases of the mechani" 
d an of structures in 
3 See this journal for October, 1888. 
