288 



MINERALS AND GEOLOGY 



FIG. 233. 



FIG. 234. 



Fie. 231. FIG. 232. 



Explanation of figures : 



Fig. 231. Orthoceras Lamarcki (Billings). Calciferous Formation. 



Fig. 232. 0. (Endoceras) proteiforme (Hall). Trenton Forma- 

 tion. 



Fig. 233. 0. (Ormoceras) crebriseptum (Hall). Hudson River 

 Formation. 



Fig. 234. Part of si'phuncle, natural size, of 0. (Huronia] vertebrale. 

 Niagara Formation, Lake Huron. 



Ammonoidea : This Order, separated of late years, only, from the 

 Tetrebranchiata^ comprises a number of extinct forms, more especially 

 characteristic of Mesozoic formations, although including Jbwo Palae- 

 ozoic genera, Clymenia and Goniatites. In most forms the shell is 

 convolute or closely enrolled ; but straight, curved, spiral and other 

 shapes are occasionally presented, especially by Cretaceous genera. 

 In all, the shell is divided into chambers by septa, which are slightly 

 undulated in Clymenia ; more prominently undulated or indented in 

 Goniatites; and, as a rule, highly complicated with numerous lobes 

 or foliations in the Ammonites proper. The siphuncle is small and 



