317 
stage the sides of the shell become more flattened, and the abdominal 
shoulders squarer, the varices frequent, and strong intermediate ribs ap- 
pear on the sides and abdomen. 
In the next (Cosmoceras) stage “the ribs no longer cross the abdo- 
men, but end in tubercles on the abdominal shoulders, forming well de- 
fined shoulder keels, with a furrow between them.” Near the beginning 
of the fourth coil the ribs are reduced to mere faint undulations and fine 
sickle-shaped striae on the sides of the umbilicus, while the external tu- 
bercles become almost obsolete, forming mere notches on the continuous 
abdominal keels. Specific characters begin to appear here. This may be 
taken as the beginning of the Hoplites stage. The septa have not reached 
the complete development of the genus. 
The umbilical knots begin at this stage, and growing stronger, become 
a characteristic feature of the adult Placenticeras. “Placenticeras pacifi- 
cum at this stage is wholly unlike P. californicuwm, with which it is asso- 
ciated, being much more compressed and discoidal, with narrow abdo- 
men, flatter sides, much less distinct sculpture, and narrower umbilicus, 
although in the earlier adolescent periods both species are very much 
alike.” The shell passes from this stage by gradual changes into the adult 
Placenticeras. 
Professor Smith’s conclusions are of especial interest. He says: ‘The 
development of Placenticeras shows that it is possible, in spite of dog- 
matic assertions to the contrary, to decipher the race history of an animal 
in its individual ontogeny.” 
‘for further illustrations of recapitulation among the Cephalopoda, the stu- 
dent should consult the following papers: Branco, W., Beitriige zur Hntwicklungs- 
geschichte der fossilen Cephalopoden, Paleontographica, vols. xxvi, xxvii, 1879, ’SO. 
Buckman, 8S. 8., Monograph of the Inferior Oolite Ammonites, Paleontographical 
Society, 1887-'96. Hyatt, A., Paralellism of the individual and the order among 
tetrabranchiate Moliusks, Mem. Bos. Sec. Nat. Hist., vol. i, 1866; Fossil cephalo- 
pods of the Museum of Comparative Zoology, Bull. Mus. Comp. Zool,, vol. iii, 1872; 
Genesis of the Arietidxe, Smithsonian Contr. to Knowl., vol. xxvi, 1889; Phylogeny 
of an acquired characteristic, Proc. Am. Phil. Soc., vol. xxxii; Cephalopoda, in 
Test Book of Paleontology by Zittel (HMastman trans.), 1899. Hyatt, A., and Smith, 
J. P., Triassic cephalopod genera of North America, U. S. Geol. Surv. Prof. Paper 
No. 40, 1905. WKarpinsky, A., Ueber die Ammoneien der Artinsk-Stufe, Mem. Acad. 
Sct. Imp. St. Petersburg, vol. xxxvii, No. 2, 1889. Neumayr, M., Die Ammoniten 
der Kreide und die Systematik der Ammonitiden, Zeitschr. der Deutch. Geol. Ges., 
1875; Ueber unvermittelt auftretende Cephalopodentypen im Jura Mittel-Europas, 
Jahrb. d. K. K. Geol. Reichs. Wien, vol. xxviii, 1878. Smith, J. P., The development 
of Glyphioceras and the phylogeny of the Glyphioceratide, Proc. Calif. Acad. Sci., 
(3) Geol., vol. i, 1897; The Development of Lytoceras and Phylloceras, Jbid., 
1898; Larval stages of Schloenbachia, Jowr. Morphology, vol. xvi, 1899; The Car- 
boniferous Ammonoids of America, Monog. U. 8. G. S., No. xlii, 19083. Wiirtenburger. 
R., Studien tiber die Stammgeschichte der Ammoniten, Leipzic, 1880. 
