CEPHALOPODA— ORTHOCHOANITES. 93 



Strongly compressed, with narrow whorls, small umbilicus, and 

 abruptly rounded venter. Aperture with broad, rounded, lateral 

 lobes and moderate hyponomic sinus. 



Carbonic of Kansas. 



XXXVIII. Thrincoceras Hyatt. 



Nautilicones of disc-like form, with slight involution in adult 

 and no contact furrow in young; section of whorls subquadran- 

 gular; umbilical perforation large; surface longitudinally striate. 



109. T. depressum Hyatt. (Fig. 13 12.) Carbonic. 



Whorls broader than high; angles rounded. 

 Carbonic of Kentucky. 



no. T. kentuckiense Hyatt. (Figs. 1313, 1314.) Carbonic. 



Larger than preceding; height and width of whorls about equal; 

 sides flat, bounded by faint angulations. 



Carbonic of Kentucky. 



Hercoceratida. 



XXXIX. Centroceras Hyatt. 



Gyroceracones and nautilicones with slightly impressed zone 

 in later stages only, with quadragonal adult but digonai young 

 whorls (nepionic) ; later (neanic stage) the form is trapezoidal 

 in outline and furnished with tubercles, as in the young of Tem- 

 nocheilus. Devonic-Carbonic. 



111. C. ohioense (Meek). (Fig. 13 15.) Devonic. 

 Large, the whorls not impressed, but just in contact. Tubercles 



broad and low, disappearing in last portion. 

 Onondaga (Columbus) of Ohio. 



112. C. marcellense (Meek). (Fig. 13 16.) Devonic. 

 Volutions scarcely embracing in adult ; umbilical margin strongly 



angulated; sides more flattened than in preceding. Tubercles two 

 to each camera; sutures with angular saddles on inner and outer 

 margins, and broad lateral lobes; siphuncle near ventral surface. 

 (Type of genus.) 

 Marcellus of New York, etc. 



