EVOLUTION OF FOSSIL CEPHALOPODA. 243 
bryo shell, a marginal siphuncle, and a small siphonal 
lobe. This form, Bactrites, is transitional to the am- 
monoids, and may be considered as the radicle of that 
order; Plate V, Fig. 7, shows a magnified protoconch 
and early larval chambers of Bactrites. The true am- 
monoids developed out of Bactrites near the beginning 
of Devonian time; the first of these, the Mautilinide, 
differed from Bactrites in no respect except their coil, 
and Mimoceras (Plate V, Fig. 8) began its larval history 
as a straight actrites-like shell, then became curved 
like a Cyrtoceras, then loosely coiled like Gyroceras, and 
finally reached the primitive nautilian stage of involu- 
tion. Axarcestes, another of the earliest Mautlinide, 
was successively cyrtoceran, gyroceran, and finally 
close-coiled nautilian. Thus in the ammonoids we have 
a single development series, corresponding to the many 
parallel series of the nautiloids; reversionary series did 
not come in until much later in the family history. 
From the Vautilinide of the early Devonian the am- 
monoids branched out rapidly, continued increasing, di- 
verging, became highly specialized and accelerated until 
their final extinction at the end of Cretaceous time. 
Each ammonite goes through a larval history that is 
long and varied in direct proportion to the length of 
time from its period back to the Lower Devonian, 
Thus in the Vaut/inide we find very simple ontogeny, 
with no great changes from the larval up to the adult 
stages, except in the increasing involution of the later 
whorls. The higher Devonian and Carboniferous spe- 
cies go through several generic changes before they 
become adults, and Mesozoic forms have still longer 
larval and adolescent periods—that is, longer in the 
sense of having more stages. 
From the work of L. von Buch, Quenstedt, and others 
of the older paleontologists, the increasing variety of 
