Geol— Vol. I.] SMITH— PLACENTICERAS. 1 99 



Phylloceratidse, and regarded as a possible ancestor of 

 Cceloceras, Sphceroceras, and Slephanoceras. Until the 

 ontogeny of the genus is worked out, opinions on its phy- 

 logeny can not be much more than speculations. 



The more remote derivation of the yEgoceratidae is still 

 more uncertain ; Hyatt and nearly all other paleontologists 

 regard Psiloceras as the radicle of the group, while many 

 derive Psiloceras from the Phylloceratidse, the development 

 and phylogeny of which group have been very fully discussed 

 by the writer in a recent paper (1898). In his opinion the 

 ammonite radicle of this group is to be sought in Nannites, 

 or in some Permian or Triassic genus of that transitional 

 character. No attempt was made to trace the genealogy 

 back into the goniatites, further than to indicate the prob- 

 ability that the remote radicle would be found among the 

 Prolecanitidse. The ontogeny of Nannites is still wholly 

 unknown, but the writer has recently worked out the devel- 

 opment of an undescribed genus, associated with, and evi- 

 dently very closely related to Nannites^ with larval and ado- 

 lescent stages showing unmistakable glyphioceran affinities. 

 F. Bernard (1895, p. 656) derives the Phylloceratidse from 

 Popanoceras, which is most improbable, in view of the evi- 

 dent derivation of that genus from the primitive Arcestidas. 

 E. Haug (1898, p. 45) traces the Phylloceratidse through 

 Monophyllites back to JVoniismoceras and Gephyroceras of the 

 phylum Gephyroceratidse ; although, as shown by the writer, 

 the young stages of both Phylloceras and Lytoceras show an 

 unmistakable resemblance to Glyphioceras; the ontogeny of 

 jVomtsmoceras is unknown, so any attempt at present to trace 

 its origin must be largely speculative; it may, however, 

 have come from Glyphioceras. 



Whatever the goniatite radicle of this group may have 

 been, the secondary radicle of Hoplites, Desmoceras, y£go- 

 ceras, Perisphinctes, and Cceloceras was the same for all. 

 The young stages of Hoplites, as figured by Branco (1879, 

 PL XIII, fig. 2, a-m), resemble the young of Perisphinctes 

 (Branco, 1879, P^- XIII, figs, i, a-l) , while both resemble 

 the larval and early adolescent stages of Cceloceras, as illus- 

 trated on Plate XII of Branco's memoir, and of Desmoceras 



