222 MEEKOCERAS, CLYDONITES, CRLOCER AS. 



the young, the characteristics of the superior lateral cells, which 

 are invariably divided, as are those of all the Ammonites proper, 

 and also in the tendency of the young sutures of Buchiceras 

 bilobatum to assume a wholly ammonitic aspect. The truly am- 

 monitic outline of the cells and lobes in Buchicera* attenuatum 

 shows how easily the outlines of the typical divided cells are 

 transformed into those of a true Ammonite by a tew (limitations. 

 whereas the same digitations applied to the entire outlines of a 

 true Geratite would produce only a Ceratite, not an Ammonite. 



Genus MEEKOCEEAS, Hyatt. 



Distinguished from Ceratites by having but three distinct 

 lateral cells and two lateral lobes, besides the finer auxiliary 

 lobes and cells. The typical Ceratites have at least four distinct 

 lateral cells and lobes besides the auxiliary ones, and the dis- 

 tinction is slight between the two series ; in this genus, on the 

 contrary, the auxiliary series, when present, is not divided from 

 the third lateral cell by a distinct lobe, as in Ceratites, and the 

 aspect of the third lateral cell is often like that of a Goniatites. 

 The compressed whorls of all the species is of course a charac- 

 teristic which is obvious when they are contrasted with typical 

 Ceratites, as is also the absence, or merely transient appearance. 

 of heavy nodes and ribs, except perhaps in the least involute 

 species. 



Jurassic to Triassic. S. E. Idaho. 



Genus CLYDONITES, Hauer. 



Shell spiral, discoidal, whorls involute; sutures simply lobed, 

 the lobes pointed. 



Twenty-one species. Upper Triassic. Europe, Himalayas. 



Two species. Upper Cretaceous (described by d'Orb. as 

 Ceratites). Difference from Ceratites is the lobes being simple. 

 not crenulated. 



C. COSTATUS, Hauer. PI. 100, fig. 520. 

 C. DELPHINOCEPHALUS, Hauer. PL 100. lig. f)21. 



Genus CRIOCERAS, Leveille. 



Shell discoidal, Spiral ; whorls separate ; lobes foliated. 

 Thirteen species. Xeocomian to U. drreensand. Europe. 

 Some of the species have been ascertained to be merely in- 



