ASPECTS OF ONTOGENY IN AMMONITE EVOLUTION 141 



More recently, he proposed to call it by the name of "saltative 

 palingenesis";^ Dr. W. D. Lang suggested the simpler term 

 " lipogenesis "^ but as this term may convey a wrong idea of the 

 skipping of stages in ontogeny, Mr. Buckman has now proposed 

 the term "lipopalingenesis."^ 



The study of fossil lineages affords a ready means of investigat- 

 ing the various ways in which the skipping of stages has been 

 brought about. This is particularly true in the case of ammonites, 

 among which it is comparatively easy to study the ontogeny of 

 successive members of a lineage. 



From such studies it appears that there are several slightly 

 different ways in which phylogenetic stages come to be skipped in 

 the development of later members of a series. The object in this 

 article is to draw attention to some of these aspects of lipopalin- 

 genesis. 



THE OMISSION OF EARLIER CHARACTERS 



In many cases the characters omitted are those of the earliest 

 ancestors, and the "skipping," means rather the omission of the 

 earliest phylogenetic stages. This occurs as a direct result of the 

 acceleration of development (tachygenesis). A simple example 

 may be found in the evolution of the Ammonoids or Nautiloids. 

 It is generally accepted that these have evolved from straight 

 Orthoceras-Uke forms, through curved Cyrtoceras-like forms, to 

 closely coiled forms like Nautilus itself. Now it may be admitted 

 that the Orthoceras-like stage is repeated in an abbreviated manner, 

 in the early portion of the Cyrtoceras form, but in succeeding 

 members of the series this stage becomes progressively shorter, 

 and in Mesozoic Ammonites it can scarcely be said to be represented 

 at all. 



In most cases of lipopalingenesis the stage or character omitted 

 is not the most primitive.'^ This is illustrated by the skipping of 



' S. S. Buckman, "Yorkshire Type Ammonites," Vol. I (1909-12). 

 ^ W. D. Lang, Proc. Geol. Assoc, Vol. XXX (1919), p. 60. 



3 S. S. Buckman, "Type Ammonites," Vol. Ill (1920), p. 11. 



4 It should be noted that in the case just given, the skipping of the Orthoceras 

 stage merely represents the skipping of the earliest skeletal stage. 



