1^6 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



This genus in its typical form appears first in the Lower 

 Jura, but its forerunners are known in the Trias; Zittel^ 

 says it developed out of Megaphyllites, which he assigns to 

 the Cyclolobid^e ; Steinmann^ agrees with Zittel in this, 

 and further derives the family Phylloceratidse from the 

 goniatite group Prolecanitidse. J. F. Pompeckj ^ says that 

 Phylloceras in its ontogeny goes through stages correspond- 

 ing to Megaphyllttes and Monophyllites, subgenus Mojsvar- 

 ites, basing his statements on the drawings of Branco. 

 The latest published opinion is that of E. von Mojsisovics \ 

 who derives Phylloceras through Phacophyllites from Mojs- 

 varites; he says that Megaphyllttes belongs to a group with 

 accelerated development betokening degeneration, and that 

 it, with its closed umbilicus, could not have been the 

 ancestor of Phylloceras with open umbilicus. From all this 

 it may be seen that there is almost as little agreement about 

 the derivation of Phylloceras as of Lytoceras. 



The only ontogenetic studies yet published on the Phyl- 

 loceratidee have been made by Branco^, on Mega-phyllites 

 insectum Mojs., protoconch and four septal stages {of. ctt., 

 PI. VII, fig. 4) ; M. humile Mojs. (PI. VIII, fig. i), proto- 

 conch; Phylloceras heterophylluni Sowerby (PI. IX, fig. 

 i), protoconch and part of the larval stages; P. tatricum 

 (fig. 2), protoconch; Rhacophyllites tortisulcaUis d'Orb. 

 (fig. 3), protoconch and early larval stages. On these few 

 data have been based all the speculations as to the phylog- 

 eny of Phylloceras, in so far as they have been based on 

 ontogenetic study at all. Branco and most of the other 

 writers on this subject have assumed that the protoconch is 

 the important stage in ontogeny, and have neglected the 

 later stages. But the protoconch is a fixed stage, remark- 

 ably constant in all the later ammonoids, while what Hyatt 

 calls epembryonic stages, and especially the larval and early 

 adolescent stages, are really the important ones; they show 

 most distinctly the characters of ancestral genera. 



1 Handbuch d. Pal., Bd. II, 1885, p. 436. 

 - Klemente d. Pal., 1890, p. 4:8. 



3 N. Jahrb. f. Mineral, Bd. II, 1895, pp. 19 and 22, " Ammoiiiten des Rhat." 



4 Denk. K. Akad, Wiss. Wien, Bd. LXIII, 1896, p. 93, "Ceph. Ob. Trias des Himalayas." 

 ' Palseontographica, Bd. XXVI, " Beitr. Eutwick. Foss. Cephalopoden." 



