1140 The American Naturalist. [December, 
ment. During the nepionic stage the byssus becomes enclosed by 
progressive deepening of the byssal sinus and circular growth of 
the right valve. In later growth extensive resorption of the shell 
progressively enlarges the byssal foramen. The right valvegof 
Anomia glabra is thinner than the left valve, and differs also in 
being almost entirely composed of prismatic tissue. 
In the section * Studies of a Few Other Genera” the young of 
Pelecypods more or less remotely separated from the Aviculide 
are considered. In these genera the umbos of the prodisso- 
conch are directed anteriorly relatively to the larval anatomy, 
excepting in Nucula, Tellina, and the Unionidz, in which last 
group umbos are not developed. In all genera excepting the 
Unionide the nepionic stage does not have a prismatic layer. 
In genera of the Aviculide and allies, on the contrary, we find 
the umbos of the prodissoconch are directed posteriorly relatively 
to the larval anatomy (Fig. 1), and the nepionic stage has a layer 
of prismatic tissue, although this may disappear in later growth. 
In genera of the Mytilidae and Arcidz we find a well-defined 
prodissoconch and nepionic stages, which are traceable to ancient 
fossil representatives of the families. In three genera of the 
Unionidz studied the completed prodissoconch is of the same 
size and form as the glochidial stage. In structure it has not 
progressed beyond a modified condition of the phylembryonic 
stage. It is a specialized type, owing its peculiarities appar- 
ently to parasitism and correlated larval adaptation. A prodis- 
soconch and nepionic stages are described in Echinochama, 
Spherium, Petricola, Venus, Tottenia, Scrobicularia, and Saxi- 
cava. The prodissoconch is mentioned without description in a 
few other genera. In Mya arenaria a considerable description is 
given of the byssal habit, development of the siphon, and the 
shell. These present features of phylogenetic interest. 
In the development of the shell of Pelecypods the phylem- 
bryonic stage has a straight hinge line, which apparently repre- 
sents a primitive condition common to the whole class. This 
form appears to be the natural mechanical outcome of its deriva- 
tion from a primitive univalvular ancestral type. The next step 
in the development of the Pelecypod shell is the curving of the 
