ii \i.'>rc>iA. 791 



Family EUDOCERATUXA! 



Tin- -nmp was erected by Hyatt to include ortltocerau shell* having a greatly 

 rninpii'-soil form. broad lol>es ami narrow saddles, with transverse section fusiform 

 ..r -nlitriaiiu'iilar. The family was designed by its author to embrace the genera 



Kii'l i Hall) Hyatt. '/';////-, r.jv. Hyatt. i'.<l<i{>ltocrras, Hyatt, an. I /.'//. Ai/,,/,,/s-. Meek 



ami Worthen. 



Genus TKHTnt'KKAS. Hyatt. KS8. 



Compressed orthoceran -hells with broad ventral ami dorsal lolws and acute 

 lateral saddles; sipho ventral. The shell may be slightly arcuate but is usually 

 -traii_'ht at maturity: in transverse section subtriangular. 



TRIPTOTERAS PLANOCONVRXUM Hull. 1SI51. 



I'l.ATK I. VI. KIU J: PLATE I.VII. HI. 1 



Ortkoeera* planoeonv&mm MAUL, IW5I. IU-|>1. Supt. (ii-ol. Surv. Wis.,p.47. 



Orthocenu planoconrfrum WlllTFIELD, 1881 Ueolivy of Wisconsin, vol. l, p. SZ8, pi. vii. lltf. 14. 



Oriyinul // .";//)/ /</: "Shell of medium size, gradually expanding from the 

 apex toward the outer chamber, plano-convex: transverse section semicircular or 

 subtriangular, the diameters as five to nine. The convex side a little depressed on 

 each side of the middle, the opposite side nearly flat, the edges abruptly rounded. 

 Septa moderately concave, arching upwards on the sides, somewhat closely arranged, 

 about five in half an inch. Siphuncle small, central. A specimen of the outer 

 chamber, apparently of this species, is a little more than two and a half inches in 

 length, one inch and an eighth in width, the short diameter being half an inch; the 

 septa are about one-tenth of an inch distant." 



A rather small but characteristic example of thin species presents the convex 

 side exposing fifteen septa in a distance of one inch, the body chamber having about 

 the same length, so far as exposed. The curvature of the septal lobes is perfectly 



,ar ami the junction <>f the septa with the lateral margins distinctly acute. A 

 fragment of a mm-h larger imli\ idual has a body chamber measuring 60 mm. in 

 length and ."ii; mm. in width near the aperture. Tn thi- fragment are attached three 

 air-chambers the last exposing a clean septa 1 xirface and showing the ventral position 

 of the sipho. The specimen show- that while the lateral saddles appear to be acute 

 when viewed from the dorsal side, they are actually somewhat obtuse, the obtu-i-- 

 ness of the angle being distinctly manifested only on the ventral surface. A lim- 

 drawn from one lateral angle to the other shows that the dorsal convexity of the 

 shell is about twice the ventral. 



