PALAtONTOGENY AND PHYLOGENY 519 
ther divided into Zvop:tide, with long body chamber, and Cera- 
titide, with short chamber. But neither of these groups is mon- 
ophyletic, for it is quite probable, judging from their ontogeny, 
that members of both groups are derived from the Gomattide, 
and others from the Prolecanitide. Further, the authorities agree 
in deriving the 7vopitide from the Glyphtoceratide, but the larval 
stages of some of the Zvopfittde show the undivided ventral 
lobe and an unmistakable resemblance to certain Prolecanitide ; 
other so-called Tvopitid@ show the divided ventral lobe at an 
early age, and a decided resemblance to the stock of Glyphio- 
ceratide. 
Inthe same way most authorities agree that the Trachyostraca 
were all extinguished at end of the Trias, and that all the Juras- 
sic and Cretaceous ammonites, with the exception of Lytoceratide 
and Phylloceratide, were derived from the radicle Pszloceras, and 
this, too, in spite of the fact that many of the genera are rough 
shelled, and in their larval stages show marked likeness to trach- 
yostracan genera. Any naturalist can convince himself of this by 
looking at the young stages of Jurassic ammonites figured by 
Quenstedt.* Quite recently Professor W. Waagen?’ has called 
attention to the likeness of certain Trachyostraca to Jurassic 
genera, and indicated the probability of genetic relationships. 
But Mojsisovics3 says that these similarities have nothing to 
do with relationship, but are purely ‘‘convergence phenomena,” 
whatever that may mean. Resemblance of adults of Triassic 
and Jurassic forms might with some reason be ascribed to this 
mysterious agency, but surely no biologist would thus explain 
away the resemblance of larval and adolescent stages of Juras- 
sic ammonites to adult Trachyostraca of the Trias. There 
was some excuse for such opinions as long as the fauna of 
the upper Trias was not well known, and there was apparently 
a ereat, break in the Series, of ammonites. But -after the 
appearance of the monographs of G. von Arthaber, Diener, 
tAmmoniten des Schwabischen Jura. 
? Pal. Indica, Salt Range Fossils, Vol. II, p. 122. 
3 Das Gebirge um Hallstadt, Bd. II, p. 265. 
