H yatt.] 362 [June 7, 



most decidedly, as might be anticipated from the adult characteris- 

 tics, that the ancestral forms are to be sought in the Lytoceras and 

 allied groups, not in Psiloceran forms of the Trias. Coeloceras Pettos 

 is equally of uncertain derivation, though its affinities in every re- 

 spect show also that it belongs to the Dudressieri series. 



All of these forms are included under the name Stephanoceras, 

 and thus two great groups of Ammonites, the round abdomened and 

 the keeled groups, with distinct systems of development and uncer- 

 tain derivation are made to appear as one genetically connected 

 series. This, however, would not justify the total suppression of the 

 name iEgoceras and the substitution of another for the more limited 

 group, to which it can be properly applied. Scientific courtesy, as 

 well as the strict law of custom, forbids such a course, though here, 

 as in the Arietidoa, I must consider the name as used by Waagen 

 utterly devoid of zoological meaning. The structure of the Aptychus 

 has, no doubt, some meaning, but it alone certainly cannot unite Psil. 

 planorbis, JEgoc. angulatus, Coeloceras Pettos, Microderoceras Birchii, 

 etc., into one genus, because as Waagen himself points out, it has the 

 same structure in two other groups, Arietites and Amaltheus, de- 

 scribed by him as distinct genera If he had joined all these into one 

 group and distinguished them by the Aptychus, it would have been 

 more consistent and less objectionable; this characteristic would have 

 at any rate applied to them all. 



I allude particularly to this fact because the other characteristics 

 given by Waagen are not applicable to such large groups. Thus in 

 the lower forms of the Arietidae (that is to say, my genera Psiloeeras, 

 Caloceras and Vermiceras, including the planorbis, raricoslafus and 

 Conybeari series), the length of the living chamber, one of Waagen's 

 distinctive characteristics, is generally over one volution. In the 

 genus Arnioceras, the falcaries series, its length is generally less than 

 one volution, from one half to nearly a full volution. In Coroniceras, 

 from one-half to one. In Asteroceras obtusum the length is from one- 

 half to five-eighths of a volution in large specimens, in Asteroceras 

 Brookii about three-fourths. In Agassiceras Icevigatus, five-eighths 

 to three-fourths of a volution, in Agassiceras Scijjionianus, about 

 three-fourths. Thus in all the higher genera of the same family it 

 is less than one volution, and so variable that it cannot be very use- 

 fully employed, even as a specific characteristic in some species, such 

 Aster, obtusum. 



The outline of the mouth has been long used to designate sub- 

 groups among the Ammonites. This characteristic, like all others, is 



