CEPHALOPODA— AMMONOIDEA. 



173 



growth lines, with periodic narrow grooves of the same form, from 

 6 to 9 in the last volution. 



Horsetown of California; Nanaimo of Vancouver. 



Fig. 1 42 1. Pleuropachydiscus hoff- 

 mani, ^ natural size. (After Gabb.) 



Fig. 1422. Pleuropachydiscus 1 

 niani, suture. (After Gabb.) 



CXXX. Pachydiscus Zittel. 



Ventricose, thick shelled, often gigantic (.5 to i meter) ammo- 

 nites with rounded venter; with more or less well developed, 

 strong, simple or bifurcating, sometimes noded ribs, generally 

 obsolete on large individuals; constrictions faint only on younger 

 stages ; suture similar to Desmoceras, but less finely incised. Co- 

 manchic-Cretacic. 



290. P. brazoensis (Shumard). Comanchic. 

 Large (ij ft.); last whorl strongly rounded on venter, sides 



gently convex with 10 or 11 broad, slightly prominent, convex 

 ribs, which become obsolete towards the venter and umbilicus; 

 umbilicus deep, its width less than that of the last whorl ; in the 

 young the volutions are thicker at umbilical margin, the section 

 being subtriangular ; in adult the section is semielliptical, higher 

 than wide. 



Lower Washita of Colorado and Oklahoma ; Kiamitia and basal 

 Duck Creek and Lower Georgetown of Texas. 



291. P. otacodensis (StoHczka). (Figs. 1423, 1424.) Cretacic. 

 Whorls narrowing ventralwards ; umbilicus moderate, sides 



