CEPHALOPODA— ORTHOCHOANITES. 



53 



central ; living chamber three times as large as its basal diameter ; 

 septa thin, smooth, their arcuation 125°. 

 Hamilton beds of New York. 



33. 0. (Geisonoceras) leander Hall. Devonic. 

 Tube rapidly enlarging; body chamber constricted at aperture. 

 Chemung of New York and Pennsylvania. 



34. 0. indianense Hall. Mississippic. 

 Small, straight and slender ; apical angle 6° ; living chamber 



Fig. 1253. Orthoceras eriense, 

 X'A- (After Grabau.) 



Fig. 1254. a, Orthoceras epigrus ; b, O. 

 rushense ; c, Trematoceras ohioense^ showing 

 the nodes ; d^ section of same. ( b after Ind. 

 Surv., the others after Whitfield.) 



gradually enlarging except for constriction at about one third its 

 length from aperture; septa thin, moderately concave; chambers 



