22*2 MKKKOCKKAS. CLVDOMTKS. ( H I OC KKA S. 



the young, tlu- characteristics of the superior hit mil cells, which 

 are invariably divided, us are those of all t he A mmoiiites proper, 

 and also in the tendency of the young sutures of llnrh iccra^ 

 bilobatum to assume a wholly aniinonit ic aspect. The truly am- 

 monitic outline of the cells and lobes in Buchicera* (itlrnnulum 

 shows how easily the outlines of the typical divided cells are 

 transformed into those of a true Ammonite ly a lew dictations, 

 whereas the same (limitations applied to the entire outlines of a 

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



Genus MEEKOCERAS, Hyatt. 



Distinguished from Geratites by having but three distinct 

 lateral cells and two lateral lobes, besides the liner auxiliary 

 lobes and cells. The typical Geratites have at least four dist inc; 

 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 Geratites, and tin- 

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

 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. /->/v>y,r. Ilimalaijd*. 



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

 I't'i-atites). Pill'erencc from Ceratite* is the lobes being simple. 

 not creiiulated. 



0. COSTATUS, llauer. PI. 100. lig. .~>2(). 

 C. DELPHINOCEPHALUS, Hauer. PI. KM), lig. 521. 



Genus CRIOCERAS, Lcvoillr. 



Shell discoidal. spiral; whorls separate; lobes foliated. 

 Thirteen species. Neocomian to 1 T . ( \ reensand. /-Jt/ro/H'. 

 Some <>f tin- species have been ascertained to be merely in- 



