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 (Cosmoccras) 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 paclfi- 

 cum at this stage is wholly unlike P. calif or nicum, 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." 1 



1 For further illustrations of recapitulation among the Cephalopoda, the stu- 

 dent should consult tbe following papers : Branco, W., Beitrage zur Entwicklungs- 

 geschichte der fossilen Cephalopoden, Paleontographica, vols, xxvi, xxvii, 1879, '80. 

 Buckman, S. S., Monograph of the Inferior Oolite Ammonites, Paleontographical 

 Society, 1887-'96. Hyatt, A., Paralellism of the individual and the order among 

 tetrabranchiate Moliusks, Mem. Bos. Sac. Nat. Hist., vol. i, 1866 ; Fossil cephalo- 

 pods of the Museum of Comparative Zoology, Bull. Mus. Comp. Zool., vol. iii, 1872 ; 

 Genesis of the Arietidse, Smithsonian Gontr. to Knowl., vol. xxvi, 1889 ; Phylogeny 

 of an acquired characteristic, 1'roc. Am. Phil. Soc, vol. xxxii ; Cephalopoda, in 

 Text Boole of Paleontology by Zittel (Eastman trans.), 1899. Hyatt, A., and Smith, 

 J. P., Triassic cephalopod genera of North America, U. S. Geol. Surv. Prof. Paper 

 No. 40, 1905. Karpinsky, A., Ueber die Ammoneien der Artinsk-Stufe, Mem. Acad. 

 Set. Imp. St. Petersburg, vol. xxxvii, No. 2, 1889. Neumayr, M., Die Ammoniten 

 der Kreide und die Systematik der Ammonitiden, Zeitschr. der Dcuich. Geol. Ges., 

 1875 ; Ueber unvermittelt auftretende Cephalopodentypen im Jura Mittel-Europas, 

 Jahrb. d. K. K. Geol. Reichs. Wien, vol. xxviii, 1878. Smith, .T. P., The development 

 of Glyphioceras and the phylogeny of the Glyphioceratidse, Proc. Calif. Acad. ScL, 

 (3) Geol., vol. i, 1897; The Development of Lytoceras and Phylloceras, Ibid.. 

 1898 : Larval stages of Schloenbachia, Jour. Morphology, vol. xvi. 1899 ; The Car- 

 boniferous Ammonoids of America, Monog. TJ. S. G. S.. No. xlii. 1903. Wiirtenburger, 

 R., Studien iiber die Stammgeschichte der Ammoniten, Leipzic, 1880. 



