130 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



mations, or in survivals of the old types in later times. At 

 first sight it seems a hopeless task ever to work out the 

 family history of the ammonites; for, as Quenstedt once 

 remarked, a man might hope to count the stars of the 

 heavens, or even at a venture the sands of the sea, but the 

 species of ammonities are beyond his ken. But it is just 

 this infinity that gives hope of success; the grading of one 

 form into another by imperceptible steps affords an accurate 

 check on the results of ontogenetic study. And it is quite 

 clear, too, that a few genera furnish the key to the puzzle, 

 since in the later formations the majority of forms were ret- 

 rogressive and only a few remained simple and progressive. 



Because of the interest in Lytoceras and Phylloceras as 

 persistent t3^pes and progenitors of many genera of ammon- 

 ites in the Jura and Cretaceous, this investigation was 

 undertaken to find out their relationship with each other, 

 and with contemporary genera.^ If a series of species of 

 Lytoceras and PhyUoceras could be found in all the suc- 

 cessive beds of the Jura and Cretaceous it would be com- 

 paratively easy to establish in this way a genetic series. 

 But, as Neumayr" has shown, they appear sporadically in 

 successive zones, lacking entirely in many, and not forming 

 a genetic series even where they are known. They must, 

 therefore, have been immigrants from some outside region, 

 supposed by Neumayr to have been the ancient Mediterra- 

 nean sea, or " Thetys " of Suess. 



Since it is not possible to get a genetic series of adults, 

 the next best thing is to take some representative species 

 and work out its ontogeny. " The ^4 m?^?^;;^/^^^ preserve 

 in each individual a complete record of their larval and 

 adolescent history, the protoconch and early chambers 

 being enveloped and protected by later stages of the shell; 

 and by breaking off the outer chambers, the naturalist can 

 in effect cause the shell to repeat its life history in inverse 



1 The writer has been engaged for a number of years in studying the ontogeny of 

 various species of ammonites, representing the most important genera, in comparing 

 ontogenetic stages with preceding forms, and thus in working out the phylogeny of the 

 ammonites. 



- Jahrb. K. K. Geol. Reichsanstalt, Wien, Bd. XXVIII, 1S78, pp. 58-59. 



