MUSEUM OF COMPARATIVE ZOOLOGY. 71 



Goniatites, but not by any means of the simplest Goniatites. The 

 simplest adult Goniatites have no proper lateral cells, but only broad 

 lateral simple curves to the septa, as if the first septum of the Ammonite 

 was modified or broken by a small abrupt lobe on the abdominal 

 side. Contrast this with the development of the septa, and their 

 gradual change in Goniatites compressus, and we see at once that the 

 development of the same parts is very much quickened or accelerated 

 in the typical Ammonite. 



That this acceleration of development is due to the prepotency of 

 the same progressive tendency as the closer and closer coiling, and 

 final involution of the ovisac, by the first whorl, can hardly be doubted. 

 Thus, not only in the whole series of Nautiloids are the forms more 

 or less completely coiled and finally enveloping, but in the young 

 Ammonoids this process is repeated, but only as a reversionary ten- 

 dency of individuals and species, or at most, perhaps, by the group of 

 Nautilini. 



In the Arietida?, and many other groups of typical closely coiled 

 Ammonites, the same process is repeated to a greater or less degree in 

 nearly every series of species, the progress being from a non-involute, 

 or slightly involute, to a more involute form, and even in varieties of 

 species there is occasionally a marked difference in the degree of in- 

 volution of the adult whorls. Everywhere, throughout the order of 

 Ammonoids, we meet with this constant repetition without reference to 

 the geographical position or distribution of the species. 



This increase of involution is, of course, due to the extension of the 

 sides of the whorls inwardly, and is invariably accompanied by a 

 decrease in the lateral or transverse diameter of the whorl, flattening 

 of the sides, and a corresponding elevation of the abdomen. 



In those series, however, such as the Dactyloida? and Cycloceratidoe, 

 in which the amount of involution remains the same throughout, the 

 highest species, such as Dactylioceras Braunianum, and Cycloceras 

 iEgion, or Masseanum, have flattened sides and acute abdomens. 

 These modifications, being the same as those which are correlative with 

 the increasing involution of the species in other" series, produce, also, 

 mimetic forms which only need one characteristic, that of involution, to 

 become closely representative of the deeply involved species. 



Thus, all the typical Ammonites may be resolved into natural series, 

 in which the different forms in each series are related to each other 



