458 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Genus PHYLLOCERAS, Suess. 



Synoa. — Ammonites (sp.), of authors ; not of Brug., as restricted. 



Ph'jlloccras, Suess (1865), der k. k. Akad. der Wissenscb., Wien,LII,76.— Zittol (1868;, Pakeout. 



Mittheil. aus dem Mus. des kiiuigl.-bayer. Staats.,50 and 153. 

 Rhacoccras (Agassiz, MS.), Hyatt (1868), Bulletin V, Cambridge Museum of Compar. Zoology, 



86. 



Etyr.i. — QvAhov, a leaf; acpac, a horn. 

 Type. — Ammonites lietcrophyllus, Sowerby. 



Shell discoid ; periphery rounded ; volutions compressed, each embrac- 

 ing from about two-thirds to the whole breadth of the next within, often pro- 

 vided with distantly-separated internal ridges, leaving constrictions on casts 

 of the interior ; umbilicus small to very small, or closed ; surface generally 

 with mere striae of growth, or small costfe ; body-chamber long; septa often 

 crowded, provided with from about six to nine lateral lobes, and as many 

 sinuses on each side, all of which diminish regularly in size from the periph- 

 eral to the umbilical margins; lobes much branched, generally all tripartite 

 at their extremities, with alternating lateral branches, the first lateral lobe 

 being longer than the siphonal ; sinuses deeply divided and foliaceous. 



This genus was founded by its author on Jurassic forms ; Ammonites 

 heterophy/lus, Sowerby, being cited as its type. He also stated, however, 

 that it would include some Cretaceous species. Whether the latter can be 

 properly included may perhaps be a question in regard to which there will 

 be different opinions, though it seems to me that it would be rather difficult 

 to point out any satisfactory reasons why such Cretaceous species as A. Guet- 

 lardi, Raspail, A. semisulcatus, A. thety.s, A, Morelianus, A. pictwatus, and A. 

 Terverii, d'Orbigny, should be excluded from this genus. 



Phylloccras! II a 1 1 i , M. & H. 



Plate 24, figs. 3, a, b, c. 

 Ammonites Halli, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Pbilad., 70 ; and (1860) ib., XII, 420.— 

 Meek (1804), Smithsonian Check-List N. Am. Cret. Fossils, 24. 



fig- 64. Shell attaining a 



rather large size, mod- 

 >vS erately compressed- 

 discoidal ; volutions 

 with their convexity 

 about equaling two- 

 thirds their diameter 

 from the ventral side 

 to the rather narrowly 

 rounded periphery, in 

 young and medium-sized examples, each embracing nearly the entire breadth 



A septum of PhyVoccras ? Halli (two diameters), given to show some of 

 the details more accurately than figure 3c ou plate 24. 



