|, A 



.-an .-sted i- tin- M.me\\!iat greater interval between tin- lamell.i- .in. I tl.i-ir 



Wronger \rntr.il curvature in tin- ( ':ui.i<li;in example. 



Formation <m<i l^-alitf. Trenton lliu tin 1 vi<-.' 



.'. !i - 



t'lKTOCKKAS HOUOHTONI, Sj>. HIT. 

 PLAT H-tt. 



>hell .-mall, -Imrt. slightly arcuate, very compressed laterally, the dorso-ventral 

 diameter being from one and a half to twice the lateral diameter. l>m-al siirlacc 

 i ibtusely rounded, ventnil margin su I. acutely convex. Transverse section narrowly 

 ami rather acutely ovate. Greatest diameter of the tube, ju-t Miiml the aperture. 

 a I >< ml twice that at the 16th septum. Hody-chamber moderately deep, not direct 

 luit sharing in tin- general arcuation of the shell. Septa moderately, somewhat 

 irregularly distant. Sixteen of the air-chambers have a length of Hi nun. on the 

 .-ides. with a scarcely perceptible upward curve on thedorsuin, and a broad ventral 

 saddle which may be subacutely angled. Greatest convexity at the base of the 

 body-chamber which is somewhat contracted toward the aperture. Sipho dor-al. 

 -nbmarginal. Surface covered with line -tri.e which over the body-chamber, are 

 curved backward. A specimen 39 mm. in length on the venter has a bod\ -chamber 

 15 mm. deep. In one 45 mm. in the same dimension, the body-chamber is 20 mm. 

 long. This specimen has the body-chamber entire. 



In this species the great lateral compression of the shell is the primary dis- 

 tinguishing character. This added to the arcuation of the body-chamber and the 

 very slight convexity of the septa renders the species readily separable from other 

 described forms. 



formation and tocalitg. Four -|- .-;in. i,- ' t). in the Tri-ntoii inn. -! IM .f Cannon Fall*. 



Minnesota, occur In the matt-rial loaned by the lai-- W. II 



CYRTOCBBAS PKATHKRSTOMI \n.m. >/.. uov. 



l-i.ATl I. VIII. IIM. I-U. 



11 small, -lightly arcuate, gently contracted at the aperture, expanded a 

 little on the body-whorl and tapering toward the apex in low, convex curves. 

 Dorsal or inner surf.c depressed convex, rounding rather abruptly at the 



sides to a somewhat elevated venter. Transverse section broadest laterally. 

 The greatest lateral diameter of the septum divides the major axis into parts 

 which are as 2 to 3, the greater being ventral. Kach septum is gently concave, the 

 concavity being the most pronounced on the ventral slope. Air-<-hambere closely 

 appressed, thirteen of them measuring !' mm on the dorsal side and 23 mm. on 



