98 CARBONIFEROUS AMMONOIDS OF AMERICA. 



Gastrioceras welleri Smith, sp. nov. 

 PI. XXIV, figs. 13-20. 



Form evolute, widely umbilicate; whorls low and broad, increasing 

 slowly in size. Cross section trapezoidal, ventral arch low. Umbilicus 

 deep, funnel-shaped, with abruptly angular umbilical shoulders. The 

 height of the whorl is one-third of the total diameter, and the width is 

 twice the height. The width of the umbilicus is slightly more than one- 

 third of the diameter of the shell. The impressed zone is about one-sixth 

 of the height of the whorl. The greatest breadth of the whorl is at one-half 

 the height. 



The surface is ornamented with both cross stride of growth and periodic 

 constrictions. These are seen on both shell and cast. The cross stria^ are 

 sinuous and show only imbrication of the shell. There are no crenulations 

 nor spiral lines visible The constrictions on the last Avhorl are five in 

 number, deeply incised, and slightly sinuous, bending forward in a broad 

 sinus. The cross stride also form incipient undulations on the shell and 

 cast. The length of the body chamber is at least one revolution. 



The septa are tyj^ically gastrioceran ; the ventral lobe is divided by a 

 narrow siphonal saddle into two sliort, narrow branches; the principal 

 lateral lobe is deeper, broad and pointed; the second lateral lobe is on 

 the umbilical slope, just outside of the suture. 



This species is most nearly related to iiastrioceras khigi. Hall and 

 Whitfield, but differs from that species in its weaker sculpture, fewer con- 

 strictions, less hiji-hlv arched whorls, and more angular shoulders. The 

 relative dimensions and the septa of the two species are exceedingly 

 similar. The species belongs to the group of Gastrioceras globidosum 

 Meek and Worthen, characterized by rather globose shape and absence of 

 umbilical ribs. But G. iveUeri is proportionally narrower and has a wider 

 umbilicus than G. gJobiilosum. 



Occurrence. — In the Middle Coal Measures, Des Moines formation, of 

 Carroll County, Mo., exact locality unknown. The figured specimens 

 are deposited in the paleontologic collection of the Walker Museum, 

 University of Chicago (No. 1313). The specific name is given in honor 

 of Dr. Stuart Weller, to whom the writer's thanks are due for the loan 

 of the specimens. 



