INVERTEBRATE PALEONTOLOGY. 463 



1 . placenticeras, Meek (typical). 



Shell with the very narrow periphery truncated, and often pro- 

 vided with a row of compressed alternating nodes along each margin ; 

 volutions each 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 moder- 

 ate. — (Type as already stated.) 



2. sphenodisctjs, 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 reniform at the ends. — {Ammonites lobatus, Tuomey). 



The most marked features of this genus are its lenticular form, very 



narrowly -truncated or sharply-cuneate periphery, broad, deeply embracing 



volutions, sagittate aperture, small umbilicus, and numerous, not very large 



or very deeply-divided lobes and sinuses, arranged in undulating series across 



each side, the lateral lobes increasing in size to the third one inclusive 



(which is always the longest of the entire series), and those beyond in all 



cases regularly decreasing in size to the umbilical margin. 



In addition to the type-species P. placenta, the typical section of this 

 group will include the Indian species P. Andoorensi's (= A. Ancloorensis, Sto- 

 liczka), as well as two other Indian Cretaceous forms referred by him to A. 

 Guadalouptf, Eoemer, and A. Orbignyanus, Geinitz ; also, the typical Texas 

 form of A. Ghiadaloupce, which may be distinct from the Indian shell referred 

 to it. The Texas form is perhaps an extremely aberrant example of the 

 group, having much more convex and less widened volutions, with also more 

 strongly-developed nodes than usual. The greater convexity of its volutions 

 also causes the truncation of its periphery to be much less apparent ; but 

 still it exists with its marginal rows of compressed, alternately-arranged 

 nodes; while its septa clearly agree in the number, proportions, and arrange- 

 ment of the lobes and sinuses with the more typical forms of this group. 



The Split nodiscus group, in addition to the type-species P. knticularis 

 {■=: A. lentkularis, Owen), will include apparently A. Pierdenalis* von Buch, 

 as well as a European form referred to it by Binkhorst, from the Upper Chalk 



" Von Buch wrote this name Pierdenalis, but Roemer always writes it PiedernaVt*. 



