10 



whereas in cyrtoceras the siphuncle is placed sometimes on 

 the dorsal, sometimes in the ventral margin, " and in every 

 conceivable position between these two points." * 



Both the orthoceras and the cyrtoceras are nautoloids, and 

 commence life alike in one 'respect, namely, with conical 

 nuclei or ovisacs, as distinguished from the rounded ovisacs 

 of the subsequent ammonites. 



Professor Huxley would have us infer that the ammonite is 

 a modified orthoceratite, but the present state of our know- 

 ledge does not confirm this. Monsieur Gaudry, one of the 

 great masters in this science, when writing on the ovisacs, 

 lays it down as follows : " We must admit that this difference, 

 shown so plainly in the upper Silurian epoch, is, in the present 

 state of our knowledge, an argument of weight against the 

 idea of linking together the whole creation." f Since the 

 researches of Professor Hyatt this characteristic has lost some 

 of its value ; but, although he traced in one or more genera the 

 existence of an ammonitoid nucleus, yet, in the vast majority 

 of instances, the old radical difference obtains. As Dr. Blake 

 says : " We may here learn those characters which point to 

 the origin of the forms possessing them, and any fundamental 

 distinction found will prove a bifurcation of the group. "J 

 The little cap, or ovisac, is by Sir Richard Owen called the 

 protoconch, and is a distinguishing mark of origin in the vast 

 majority of cases. 



Mr. Hyatt lays much stress on the embryological facts 

 which he considers that he has established, that every in- 

 dividual curved cephalopod began life as a straight embryo, 

 becomes curved in its growth, completes its curvature at 

 maturity, has a tendency to uncoil as it arrives at old age. 

 He finds in this life of the single creature a representation of the 

 life of the tribe, and argues that in both cases alike the growth 

 is purely natural, and, as it were, self-contained. Surely this 

 is analogy and not natural history. The tribal and the indi- 

 vidual life may thus be parallel in part only. He himself 

 says elsewhere : " We cannot say that the causes which pro- 

 duced old age, and those which in time produced retrogres- 

 sive types were identical. " 



It seems obvious, therefore, that no reliance whatever can 

 be placed on the argument from embryology. 



It is admitted that the marvellously rapid introduction of 

 new species of these two orders in the Silurian epoch is 



* Salter, Memoir by Ramsay, North Wales, vol. iii., p. 374. 

 t Gaudry, Les Enchainements du Monde, &c., p. 173. J Fossil Cephalo- 

 poda, p. 24. Science, February 8, p. 149. 



