MOLLUSC A— CEPHALOPODA. 29 



c. Siphuncle near venter XIX. larphyceras. 



c. Siphuncle near dorsum XXI. Schraderoceras. 



cc. Whorls impressed often to complete involution tttt 



ffff. Impression moderate d. 



d. Whorls laterally compressed, rapidly enlarging. 



XK. Eurystomites. 



d. Whorls dorsoventrally compressed i'', 



I''. Discoidal, whorls gradually enlarging. 



XXII. Trocholites, 



l^. Not discoidal, whorls rapidly expanding a'' 



a''. Siphuncle nummuloidal near center, surface 



often spiraled XXX. Nephriticeras, 



a.^. Siphuncle not nummuloidal J 



J. Siphuncle subveniran in adult; sides of 



whorl rounded XLV. Asymtoceras, 



J. Siphuncle ventrocentren in adult ; sides of 



whorl faintly angulated L. Diodoceras. 



l^. Whorls subquadrangular b''. 



b''. Surface spiraled XXXVIII. Thrincoceras. 



W. Surface smooth \X 



XX. Sides parallel or diverging ventrally, dorsal 



lobe V-shaped XLIX. Remeleoceras, 



XX- Sides converging ventrally, dorsal lobe shal 



low XLI V. Domatoceras. 



ffff. Impression profound to complete involution 



e. Sutures faintly lobed 2^, 



2^. Transverse diameter of adult whorls similar to 



greater than height c' , 



c^. Suture with shallow ventral lobe XXX 



XXX' Siphuncle above the middle. 



XXXV. Stearoceras. 

 XXX- Siphuncle close to venter often shown in 



lobation XLVI. Solenocheilus. 



c^ . Sutures with broad ventral saddles, lateral and 

 dorsal lobes. Whorls very gradually increasing 

 in width, slightly umbilicated. 



XXXVI. Leuroceras. 

 c^. Suture nearly straight, whorls rapidly widening* 



umbilicus closed LI. Eutrephoceras. 



2' . Whorls laterally compressed d''. 



d''- Venter rounded LI. Eutrephoceras. 



d^. Venter truncate, flat and narrow. 



XXXVII. Phacoceras. 



Sutures stron gly lobed 3''. 



3^. Ventral saddle narrow, notched. 



LI II. Proclydonautilus. 



3^. Ventral saddle broad, undivided e''. 



e''. Siphuncle small, centren or subcentren. 



LIV. Hercoglossa. 

 t^. Siphimcle large, close to dorsum. ..LV. Aturia. 



