Vol.54-] DIVISIONS OF SO-CALLED ' JUKASSIC' TIME. 455 



concentric coiling in the latter case differs from that of the first, and 

 is the result of the change brought about by a period of excentric 

 coiling. 



These stages may be seen in Ammonites Humphriesianus, d'Orb., 

 pi. cxxxiv : — 1st stage, all whorls, except the last J whorl ; 2nd 

 stage, last I whorl. In A. Humphriesianus, d'Orb., pi. cxxxiii 

 (Bayleanus, Oppel), reduced one-third: — 1st stage, about first 

 four whorls ; 2nd stage, about the next whorl, when the shape 

 of the whorl gradually changes from depressed to compressed, and 

 becomes narrower ; 3rd stage, the rest of the fossil. 



I desire to direct especial attention to these figures, because 

 some remarks of mine in this connexion appear to have been 

 misunderstood. I would also direct attention to Morphoceras as a 

 descendant of Sphceroceras, where Sphceroceras shows the beginning 

 of excentric umbilication. Morphoceras dimorphum shows ex- 

 centric umbilication with compression of the whorl, tending to 

 become again concentric, and Morph. ^ polymorphum,' (d'Orb., 

 pi. cxxiv, figs. 5 & 6) shows the further development of this renewed 

 concentricity. I claim that the same change of coiling which 

 produced Ste'pheoceras from Coeloceras, and Morphoceras from 

 ISphoeroceras^ also produced the Arietidse, etc., from Cymhites. 

 Further, that it is a mistake to regard Cymhites as a senile form 

 incapable of producing descendants because of its abnormal body- 

 chamber, and to class it with admittedly senile forms such as Cado- 

 moceras, (Ecoptychius, CEcotraustes, etc. — as grave a mistake as it 

 would be to regard the want of teeth in a baby's mouth as the same 

 pheuomenon as their absence from the mouth of an old man. 



In regard to the evolution of Stepheoceras^ it may be remarked 

 that the same method which produces the true Stepheoceras from 

 Coeloceras is repeated again and again to produce other similar 

 series as degeneratives of Coeloceras. Hitherto all these forms 

 have been called Stephanoceras ; but in reality they are hetero- 

 genetic or polygenetic series, agreeing only in the fact of being 

 in a certain stage of development — the stage when the broad peri- 

 phery gives place to the narrower one. 



Similarly, the present use of Perisphinctes means only polyge- 

 netic series of degeneratives from Stepheoceras and allied forms 

 — series which have lost the spinal ornaments. Sphceroceras is 

 really used also in a similar manner ; and it may be noted that if 

 the broad periphery be retained, but becomes arched with increase 

 of inclusion the while, forms called Sphceroceras and Emileia are 

 produced. Later, in these forms is seen the process of decrease 

 of inclusion resulting in Morphoceras, on the one hand, and Emileia 

 polymera on the other. But when this decrease of inclusion takes 

 place pari passu with the reduction of the breadth and the arching 

 of the periphery, then are produced what have been called Stephano- 

 ceras : to one series of which, the Humphriesianum-growp^ the 

 name Stepheoceras is now appended, in lieu of Stephanoceras, which 

 must lapse altogether on account of prior use. 



