Geol.-Vol. I.] SMITH— PLACENTICER AS. 197 



in the ^goceratid^, subfamily Perisphinctinae, division (b) 

 Tuberculati, along with Acanthoceras and Holcodiscus. All 

 these writers connect the genus with Psiloceras as the radicle 

 from which the entire perisphinctoid stock sprang, although 

 the opinion is based, not on ontogenic study, but purely on 

 the geologic succession of types, a satisfactory method only 

 where the paleontologic record is very complete. 



In a later paper Sarasin (1897) concludes, as a result of 

 ontogenic study, that Hoplites is not a member of the highly 

 specialized Stephanoceratidas, but comes from the more 

 primitive perisphinctoids along with Desmoceras. Most 

 writers agree that the perisphinctoids come from the ^go- 

 ceratidae, that they, in turn, are derived from Psiloceras, 

 and that this genus branched off from the Phylloceratidoe. 

 E. Haug (1894) expresses the opinion that not only the 

 y^goceratidce, but also the Amaltheidee, the Arietid^, and 

 most of the other Jurassic stocks were derived from the 

 PhylloceratidcE, because their septa are trisenidian in earl}^ 

 youth, and megaphyllian in the development of the princi- 

 ple saddles. This seems to be nearer the mark, although 

 the descent must be from some of the earlier unspecialized 

 forms of the Phylloceratidae, such as possibly Monofhyllitcs, 

 although certainly not from any known species of that 

 genus; at any rate, none of these genera show a Psiloceras 

 stage in their ontogeny. S. S. Buckman (1898, p. 445), 

 contrary to the general opinion, says that Psiloceras is not the 

 ancestor of the Arietidas nor the ^goceratid^, but is, itself, 

 a degenerate form. 



The immediate ancestors of the -i^goceratida? may be 

 sought among the Polymorphidte, a group established by 

 E. Haug (1887) to include a number of the more primitive 

 ammonites of the Lias. Agassizceras, the principal genus 

 of the group, is considered by Hyatt (1889) ^^ be the imme- 

 diate ancestor of the agassizceran branch of the Arietidee, 

 and the direct descendant of Psiloceras. Neumayr (1878) 

 segregated a number of the simpler species of this group 

 under the name of Cymhites, characterized by rather glo- 

 bose form, very slightly ammonitic septa, and absence of 



(2) August 6, 1900. 



