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 Nautilinida;, 

 differed from Bactrites in no respect except their coil, 

 and Mimoceras (Plate V, Fig. 8) began its larval history 

 as a straight Bactrites-\\V.& shell, then became curved 

 like a Cyrtoceras, then loosely coiled like Gyroceras, and 

 finally reached the primitive nautilian stage of involu- 

 tion. Anarcestes, another of the earliest Nautilinidce^ 

 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 NautilinidcE 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 NautilinidcB 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. 

 i From the work of L. von Buch, Quenstedt, and others 

 I of the older paleontologists, the increasing variety of 



