No. 409.] THE LARVAL COIL OF BACULITES. 43 
The metanepionic or second larval stage is characterized by 
the sudden change in sculpture which takes place at the end 
of the first revolution, where the pustules are replaced by fine 
cross striae and ribs parallel with these (Fig. 18). This stage 
may be arbitrarily considered to last as long as the coil con- 
tinues, but the spiral widens and at a quarter of a revolution 
beyond the constriction the shell leaves the coil and grows out 
nearly straight. With this the paranepionic stage may be 
considered to begin, and to continue so long as the septa are 
goniatitic ; at the distance of two and a half millimeters from 
the coil the septa begin to become ammonitic, and the larval 
stage ends (Figs. 19 and 20). The impressed zone continues 
in the shaft for nearly a millimeter from the coil, but before 
the paranepionic stage has ended the cross-section of the shaft 
becomes rounded instead of semilunular. The larval shell 
seems to be always unsymmetric, at least in the large number 
of specimens studied, and this lack of bilateral symmetry was 
not due to crushing, but actually to one-sided growth. The 
writer has observed that this is quite common in degenerate 
forms of ammonoids, whilé progressive species with normally 
healthy growth are exceedingly symmetric and constant in 
development. : 
The digitation of the septa, which begins with the indenta- 
tion of the first lateral saddle at about two and a half milli- 
meters from the coil, marks the beginning of the adolescent or 
neanic stage. The complexity of the septa increases slowly at 
first, but soon becomes more rapid as the whorl begins to be 
compressed laterally, which takes place at about eighteen milli- 
meters from the coil. This lateral compression may be regarded 
as the beginning of the metaneanic or second adolescent stage, 
which, however, cannot be sharply differentiated from the 
others. The angle of increase of size of the whorl throughout 
the later larval and earlier adolescent stages is considerable, 
but at the distance of about thirty millimeters from the coil 
the angle becomes smaller, indicating a distinct change in the 
rate of growth. This may be called the last adolescent or 
paraneanic stage, and forms a gradual transition to mature 
conditions of growth. No sharp line of demarcation exists, 
