Geol.— Vol. I.] SMITH— LYTOCERAS AND PHYLLOCERAS. 1 35 



Dr. R. T. Jackson has recognized the value of this prin- 

 ciple of retardation in his studies of the pelecypods/ and 

 the writer has noted it in unpublished observations on the 

 ontogen}^ of many species of ammonites. 



The law of tach^^genesis was first used by Hyatt in the 

 study of cephalopods, and has since been applied success- 

 fully to the pelecypods by Jackson; to the echinoderms by 

 Jackson"; to the brachiopods by Beecher, J. M. Clarke, 

 and Schuchert": to the crustaceans by Beecher.^ It is 

 therefore no longer a theory, but a fixed principle of 

 biology, to be used regardless of any bias towards theories 

 of natural selection or adaptation. It is, however, quite 

 true that all workers in paleontogeny are decided adherents 

 of the Neo-Lamarckian school. A summary of the princi- 

 ples and methods of ontogenetic research has been given 

 by the writer in a recent paper. ^ 



Family LYTOCERATID.E Neumayr. 



The Lytoceratidae, as defined by Zittel,^ contain forms with wide, shallow, 

 umbilici, evolute, scarcely embracing whorls, smooth except for constrictions, 

 fine striae of growth, and occasional ribs made by varices. They have a 

 divided siphonal lobe, two principal lateral, with almost no auxiliary lobes. 

 Body chamber one-half to two-thirds of a revolution. 



In this group are included species from the Upper Trias 

 to the Upper Cretaceous, most of the abnormal retrograde 

 genera such as Tiirrilites, Baculites, and Hamites being 

 placed here. Paleontologists are not agreed as to the rela- 

 tions of this family with the Phylloceratidse, some regard- 

 ing it as descended from that family, others as the ancestral 

 stock from which they sprang, and others still regarding 

 both as widely divergent branches from the Prolecanitid^. 



1 Mem. Boston Soc. Nat. Hist., Vol. IV, No. 8, " Phylogeny of the Pelecypoda," p. 381. 



- Bull. Geol. Soc. Amer., Vol. VII, 1895, " Stvidies of PalEeechiuoidea." 



3 Amer. Jour. Sci., Vol. XLIV, Aug., 1892, "Development of Brachiopoda, Part II"; 

 Trans. Conn. Acad. Sci., Vol. IX, March, 1892 ; and Proc. Biol. Soc. Washington, Vol. VIII, 

 July 13, 1893. 



•• Amer. Jour. Sci., Feb. and March, 1897, and Amer. Geol., Vol. XVI, Sept., 1895. 



^ Jour. Geol., Vol. V, No. 5, July-August, 1897. "Comparative Study of Palseontogeny 

 and Phylogeny." 



6 Handbuch d. Palseont, Bd. II, 1885, p. 440. 



