222 MEEK0CER.A8, rj.Y DONITES, ORIOCERAS. 



the young, the churacteristics of the superior hvteral eells, wliiuh 

 are invariabl^y 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 Buchiceras aUenuatum 

 shows how easily the outlines of the typical divided cells are 

 transformed into those of a true Ammonite by a few digitations. 

 whereas the same digitations applied to the entiic outlines of a 

 true CeratUe would })ro(luce only a Ceratite. not an Ammonite. 



Genus MEEKOCERAS, Hyatt. 



Distinguished from Geratites by liaving but three distinct 

 lateral cells and two lateral lobes, besides the liner auxiliary 

 lobes and cells. The typical Geratites have at least fo\ir distinct 

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

 tinction is slight between tlie 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 Geratites, 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 

 Geratites, as is also the absence, or merely transient appearance, 

 of heavy nodes and ril)S, except perhaps in the least involute 

 species. 



Jurassic to Triassic. S. E. Idaho. 



Genus CLYDONITES, Hauer. 



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

 the lobes pointed. 



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



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

 Geratites). Difference from Geratites is the lobes being simple, 

 not crenulated. 



C. COSTATUS, Hauer. PI. lUU, tig, .520. 

 C. DELPHiNOCEPHALUS, Haucr. PL 100, fig. 521. 



Genus CRIOCERAS, Leveille. 



Shell discoidal, spiral; whorls separate; lobes foli;ited. 

 Thirteen species. Neocomian to U. (xreensand. Europe. 

 Some of the sp'ecies have been ascertained to be merely in- 



