2:-5X PLACENTICERAS, SPIIENODISCUS, ETC. 



by smaller ones ; on the last, the costte and their nodes become 

 very prominent, the keel depressed and broken into a series of 

 elongated isolated nodes. 



P. WOOLGARI, Mantell. T. 100, figs. 601, 602. Cretaceous. 

 England, United States. 



Professor Meek remarks that the type of the genus so nearly 

 resembles species of Professor Hyatt's genus Pleuroceras that 

 were it not for the opinion of Hjatt that none of the Liassic 

 groups range up into the cretaceous, he would not separate 

 them. It -would perhaps be much more convenient to continue 

 to regard the various forms of Ammonites simply as sections of 

 a single genus, than to make unlimited and overlapping genera 

 without good characters. 



Genus PLACENTICERAS, Meek. Shell with the very narrow 

 periphery truncated, and often provided with a row of com- 

 pressed alternating nodes along each margin ; volutions about 

 three-fourths embraced by the next succeeding outer one ; septa 

 with the lateral sinuses provided with more or less branched and 

 digitate terminal divisions ; umbilicus small or moderate. Cre- 

 taceous. United States, India. 



AMMONITES PLACENTA, DeKay. T. 105, fig. 588. 



Subgenus SPHENODISCUS, Meek. Shell with periphery cuneate : 

 umbilicus very small ; volutions each almost entirely embraced 

 by the succeeding one; septa with the first five or six lateral 

 sinuses provided with only a few short, nearly simple, obtuse 

 divisions ; while the others are simple, and usually broadly reni- 

 form at the ends. Cretaceous. United States, Europe. 



AMM. LOBATUS, Tuomey. 



Meek thinks that some of the species of Pinacoceras, Mojsi- 

 sovics, will fall into this group ; and that that genus is too com- 

 prehensive. 



Neumayr'x Classification <>!' Ammonites. 



One of the latest systematic arrangements of the Ammonite;e 

 is that of M. Neumayr, of Vienna.* It is prefixed by an inter- 

 esting account of his predecessors ; Prof. Hyatt's classification 



* ZeitscU. Deutsch. Geol. GeselL, xxvii, 854, 



