356 me. l. f. spath ok" the [Dec. 1 9 14, 



which are very close one to the other as regards external shape and 

 yet have, the one a triphyllic, the other a diphyllic first lateral 

 saddle. It seems to me, however, that the diphyllic and triphyllic 

 saddle-endings, which are very often most difficult to distinguish, 

 are of little importance if considered in the adult alone and without 

 knowledge of their development. If ffliacophyllites uermoesense 

 Herbich is as near to Hh. stella Sow. as Wanner 1 thinks, then 

 the development of its principal saddles must resemble that of 

 fig. 3, PL L, and be thus typically diphyllic. In Euphyllites, on 

 the other hand, the development is apparently, like that of Psilo- 

 ceras, monophyllic, with triphyllic endings in the adult. The 

 resemblance of its triphyllic first lateral saddle and external saddle 

 (the latter occasionally subdiphyllic) to Hhacophyllites uermoesense 

 in the adult would thus be quite superficial. 



Euphyllites is included in what Prof. Diener (1908) called that 

 remarkable stock of transitional forms between JPhylloceras, Lyto- 

 ceras, and JPsiloceras, which proved the common origin of all 

 Liassic ammonites. He would adopt Hyatt's family - name 

 ' Pleuracanthitidae ' for this group, and look for the roots of this 

 stock in the Triassic genus 3fojsvarites. 



Now JPleuracanthites is very near to Analytoceras : in fact, 

 Wahner had called it 'a connecting link' between the families 

 JGgoceratidse ( = Psiloceratidse -f- Arietidse) and Lytoceratidse. But 

 Analytoceras afticulatum (Sow.) in whorl -shape and con- 

 strictions is again a morphic equivalent of the unknown 

 Tragopliylloceras ancestor, although characterized by an already 

 distinct lytoceratid suture. Moreover, forms such as Pleura- 

 cantliites polycycloides Wahner, and Parapsiloceras polycyclum 

 (Wahner), according to their author, must have been very near, 

 morphologically and genetically, to the ancestor of JPsiloceras 

 callipliyllum. Consequently, JPleuracanthites, Parapsiloceras, 

 and JEupliyllites are, indeed, a transitional group which with Ecto- 

 centrites, might well be separated, as Prof. Diener has suggested. 



The question now arises whether Tragopliylloceras, which has 

 morphic equivalents in Pliylloceras (ffliacophyllites) as well as in 

 Analytoceras, but the suture-development of which connects it 

 more with the Psiloceratidse, should be included in this transitional 

 family of the ' Pleuracanthitidae.' Its late geological age seems to 

 be at first sight against this union, but it will be seen that the 

 psiloceratid suture up to a comparatively late stage makes it 

 truly a primitive genus ; whereas, even morphologically, Trago- 

 pliylloceras has hitherto occupied quite a distinct position among 

 the Jurassic Phylloceratidae. It is therefore considered that 

 Tragopliylloceras can more naturally be grouped with Eupliyllites 

 and the other primitive forms, than with the typical Phyllo- 

 ceratidae. The term Pleurae an thitidae Hyatt, as suggested by 

 Diener is, perhaps, not quite suitable, since Pleuracanthites itself, 



1 Rhacophyllites uermoesense in Wahner's interpretation is too compre- 

 hensive, and includes a variety of forms which might very well be separated. 



