1883.] 305 [Hyatt. 



fore, disposed to translate this case in the same way. The artifi- 

 cial line between Ammonoidea and Nautiloidea can be placed 

 between this group and Geisonoceras, with which the shells and 

 characteristics of the transitional forms blend, or between Bac- 

 trites and Mimoceras according to the weight one is disposed to 

 give to the characteristics of the apex. 



AMMONOIDEA. 



We have already noted the characteristics of this order, but we 

 find it necessary to add the following remarks. 



In each group of Goniatitinae there are forms, with whorls 

 depressed and with outlines semilunar in section, which are simi- 

 lar to Parodiceras and Anarcestes and resemble these genera also 

 in having undivided ventral lobes. There are also in these radi- 

 cal genera and in other genera, either in the adults or in the 

 larvae, forms having simple sutural outlines like those of Parodi- 

 ceras with slight rounded saddles, and broad, shallow, angular 

 lobes. There can be but little risk of error, therefore, in assum- 

 ing that the genus Parodiceras is the organic centre of the Devon- 

 ian forms, and the immediate radical of all Goniatitinae except 

 the Primordialidae. The Primordialidae appear to be directly 

 derived from Nautilinidae, and we have not been able to trace 

 any descendants. This result shows that the genus Anarcestes is 

 probably the genetic centre of distribution, the stock form in the 

 phylum of the Ammonoidea, or the first with undeniably 

 Ammonoidean form and characteristics, the immediate radical, 

 while, as we have noted above, the group of Bactrites and Gei- 

 sonoceras are the distal radicals. 



Dr. Branco has worked out very valuable results with regard 

 to the law of concentration of development among Cephalopods, 

 but has failed to appreciate their meaning or to note the fact 

 that others had tried to explain similar results. He confirms 

 views first published by the author, that the larvae of the 

 Goniatites and Ammonites have simple sutures, and the ventral 

 lobes undivided in the early stages, and that these lobes be- 

 come divided only in later stages, and farther, that the more 

 ancient forms remain longer in the simple or goniatitic stage with 

 undivided ventrals, than the later forms. He alludes also (Pal- 



PROCEEDINGS B. 8- V. H. VOL. XXtl, 20 JABRJART, 1S84. 



