PALAZZONTOGENY AND PHYLOGENY 515 
Brachiopoda, the protoconch of Cephalopoda and Gastropoda, and 
the protaspis of trilobites. The prodissoconch is a straight- 
hinged, two-muscled, toothless, smooth-shelled, bivalve stage, 
corresponding to the primitive group of Pelecypoda. Even the 
monomyarian Ostrea goes through this dimyarian stage. Pro- 
fessor W. H. Dall? has used this and other facts in the develop- 
ment of the pelecypods, giving the most satisfactory classification 
up to this time. But from the very nature of the case, when the 
ontogeny of few living and no fossil forms is known, an evo- 
lutionary classification of pelecypods is impossible. 
Cephalopoda.—The living dibranchiate cephalopods, Octopus, 
Loligo, Spirula, Argonauta and other common forms, are incapable 
of preserving the larval stages as fossils. The only living tetra- 
branchiate genus, Vautilus, can have its larval stages preserved as 
fossils, but is one of the old unspecialized types, not having 
changed greatly since the first nautilian shell, and consequently 
having no striking changes in its ontogeny. 
The animals that are capable of giving the best proof of 
evolution are the ammonites. These branched off from the 
nautiloids at the beginning of the Devonian, continued increas- 
ing, diverging, became highly specialized and accelerated until 
their final extinction at end of the Cretaceous. 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 the Vautelinide@ are the first of the new 
stock, and their ontogeny is comparatively simple, there being 
no great changes from the larval up to the adult stages. The 
higher Devonian and Carboniferous forms go through several 
generic changes before they become adults, and the Mesozoic 
genera have still longer larval and adolescent periods, that 1s, 
longer in the sense of more complicated. 
From the work of L. von Buch, Quenstedt, and others of the 
older paleontologists the increasing variety of forms from the 
goniatites of the Paleozoic to the ammonites of the Mesozoic 
*Pelecypoda, Text-book of Paleontology, kK. A. VON ZITTEL, Revised English 
Edition, Vol. I, Part 1. Macmillan & Co., 1896. 
