176 Eeport of the State Geologist. 



branched ; among the. somewhat more recent forms the points of 

 the ramification swell out into tubercles. Later a second range of 

 interior tubercles appears, and at the same time the ribs show a 

 tendency to disappear. The interior range of tubercles, and 

 afterward the exterior range diminish in turn, and the shell be- 

 comes almost smooth. This last stage is attained in Asj>idoceras 

 cydotum. If the external w^horls of an adult specimen of this 

 species are removed so as to bring to light successively the more 

 and more elementary whorls, we see that the same individual has 

 presented successively all the preceding aspects, and even on the 

 earliest whorls we find bifurcated projections which finally dis- 

 appeared. The Ammonite then has passed in succession all the 

 stages attained in the adult state by species which formerly 

 existed. 



Metamorphoses such as these are absolutely general in the 

 group of Ammonites. The ornaments are constantly modified 

 with age and the youngest stages are always identical with the 

 adult forms of more ancient epochs. It is quite necessary, there- 

 fore, in order to determine exactly to what group an Ammonite 

 belongs, to be acquainted with all the stages through which it 

 has passed, for it frequently happens that forms notably different 

 in youth lose little by little their differential characteristics in 

 virtue of the phenomenon of convergence which in this group 

 manifests itself with a peculiar intensit}^. This embryogenic 

 method is now in current use for the study of this class, one of 

 the most important for palaeontologists ; by means of this method 

 light has been thrown on the mass of writings accumulated by 

 the former researches of both earlier and later authors, and 

 Phylogeny is now the only process of classification employed on 

 this subject. 



This order of ideas is carried even farther, and, in examining 

 the most primitive stages, we have in many cases come to know 

 in what way a determinate series of Ammonites descended from 

 the more ancient and more simple forms known as Goniatites. 

 The successive septa which, as the shell increased in size, bounded 

 the living chamber of the animal, were attached to the shell 

 itself by lines of suture^ the shape of which is very important. 

 But the sutures of the earliest septa among the Ammonites are 

 extremely simple, and bear a strong resemblance to those of the 

 Goniatites of various families. The forms of the most ancient 

 Ammonites recently discovered in the Upper Carboniferous, 

 establish precisely in the adult state the transition between the 

 two groups. 



A similar attempt has been successfully made by Jack- 

 son in regard to the Acephala. This author has seen in 

 living species the young forms of the Oyster and Pecten, 

 and has shown that these forms were provided with organs 

 such as the byssus, the anterior muscle, etc., which are want- 

 ing in the adult, and the aisappearance of which is accom- 



