JEGOCERATHVE. 251 



authors and as I have myself assumed ; but there may be ques- 

 tion of the accuracy of this reference, as the latter belong to 

 the Amalthese. Relative to the cretaceous Phylloceratidae it is 

 to be observed, that a large part of the forms described by 

 d'Orbigny were founded on young specimens, which had not yet 

 developed the specific characters and which must therefore be 

 withdrawn. In regard to some of the Indian forms described 

 as Heterophylli by Stoliczka, I am not certain that the}^ belong 

 to Phylloceras on account of the imperfect representation of the 

 lobes ; in the hot climate of India the oily layer with which the lith- 

 ographic rocks aiv covered always becomes somewhat softened, 

 so that the more minute details are often lost. 



Seventy-seven species ; Trias, Jura, Cretaceous. 

 PHYL. OCCULTUM, Mojs. T. 110, figs. 648, f>49. 



Family IV. ^EGOCERATID^. 



The forms, which can be traced from Mgoceras, present such 

 manifold characteristics, it is not possible to even offer one posi- 

 tive character, with the exception of the attached cover of the 

 nidamental gland, which indeed, has been observed only in a 

 limited number of species, but in forms belonging to most of the 

 included groups. All the forms also, which we know, have the 

 lobes toothed all around, but by which, to be sure, the possibility 

 of the existence of a stem form with simple sutures is not 

 excluded. 



The geologically oldest forms are those of the Muschelkalk, 

 the affinity of which with those of the Lias Beyrich was the first 

 to apprehend ; they are absent in the upper Trias in almost all 

 the yet known localities and again appear first in the uppermost 

 strata in ^Egoceras planorboides. At the beginning of the 

 Jurassic they attain an extraordinary development, the details 

 of which will be described. 



From the great extent of the family it is perhaps better to 

 subdivide them in the following manner : 



1. ^EGOCERATIN^E : jEgoceras, Arietites. 



2. HARPOCERATIN.E : Harpoceras, Oppelia, Haploceras. 



3. STEPHANOCERATINJE : Stephanoceras, Cosmoceras, Ancylo- 

 ceras, Baculina, Simoceras, Perisphinctes. Oloostephanus, Sea- 



