2;>S PLACENTrCERAS. isPHENODISCllS, ETC. 



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

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

 elongated isolated nodes. 



P. WooLGARi, Mantell. T. loi;. tigs. 601. 00l>. Cretaceous. 

 England^ United States. 



Professor Meek remarks that the type of the genus so nearl}' 

 resembles species of Professor Hyatt's genus Pleuroceras that 

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

 groups range up into the cretaceous, he would not separate 

 them. It Avould 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. 



Grenus Placenticeras, Meek. Shell with the YL'vy narrow 

 periphery truncated, and often pro^•ided 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 ; uiBbilicus small or moderate. Cre- 

 taceous. United States, hidia. 



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, Ewope. 



Amm. lobatus, Tuomey. 



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

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

 prehensive. 



Neumayr''s Classification of Ammonites. 



One of the latest S3'stematic arrangements of the Ammoniteje 

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

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



* ZeiiscJi. Deutsch. Geol. QeselL, xxvii, 854, 1875. 



