106 PALEONTOLOGY. 



tliem to the same. To this Professor Hyatt kindly assented, and I now give 

 his descriptions of the new genera, and remarks on some of the species; 

 placing his initials (A. II.), wherever quotations are made from his manu- 

 script. 



I should remark, however, that I had already identified among the 

 specimens, the following genera, viz., Trachyceras, Chjdonites, and Arcestes, 

 and referred the species to forms figured and described by Mr. Gabb in the 

 Palaeontology of California, ranging them under the above-mentioned 

 genera. 



It would be superfluous for me to attempt the expression here of any 

 opinion of my own in regard to the propriety of the subdivision of the old 

 genus Ammonites into such a great number of genera, and even families, as 

 is done in the new classifications, since I have never made an especial study 

 of this extensive group of shells with the view of forming an opinion on this 

 point. I can therefore only say that Professor Hyatt's conclusions have not 

 been lightly adopted; but that he has, on the contrary, devoted much time 

 and patient research to the study of one of the most extensive and complete 

 collections of this group of shells in the world. 



It may be proper for me to explain here some differences of terminology 

 that Avill be observed in Professor Hyatt's and my own descriptions and 

 remarks. For instance, he very properly describes the external margin, or 

 outer side of the volutions in the Ammonitoid types, as the abdomen, and the 

 inner as the dorsum; while I have used the term periphery for the former, 

 and umbilical or inner side for the latter. His method has the advantage of 

 properly expressing the true relations of the animal and its shell; though I 

 have rather preferred the terms I have used, because they avoid the per- 

 plexing confusion of ideas liable to arise in the minds of those who have 

 become familiar with the (until recently) prevalent method of describing the 

 outer side as the dorsal, and the inner as the ventral, as we see in nearly all 

 the published works on such shells. Again, in the same way, he terms the 

 outer lobe of each septum the abdominal lobe; while I have for the same used 

 the term siphonal lobe, already in use by some European authors. The lateral 

 lobes and intervening sinuses of the septa I have merely numbered consecu- 

 tively ^>-5<, second, third, and so on, from the outer one inward, instead of using 



