474 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



septate volutions), 10 inches; convexity of same. 2.30 inches. Young 

 examples are proportionally more compressed. 



Septa crowded, and each provided with eleven or twelve rather short, 

 not very deeply divided, lateral lohes and as many sinuses, on each side; 

 siphonal lobe twice as wide as long, and bearing two widely-separated, 

 diverging, terminal branches, each of which has three very short, palmately- 

 • spreading branchlets, which, in large specimens, show a tendency to 

 bifurcate ; first lateral sinus about half as wide as the siphonal lobe, which it 

 equals in length, while its body is very much contracted, and its anterior or 

 tree end bi- or tripartite, the division being merely broadly-bilobate, with 

 otherwise nearly simple margins; first lateral lobe nearly or quite as long as 

 the siphonal, but less than half as wide, and bipartite, the divisions being 

 short, equal, and shortly-bifid, while its narrow body also bears two short, 

 opposite, slightly-bifid lateral branches ; second lateral sinus a little longer 

 than the first, very contracted in the middle, and broadly trilobate or bilo- 

 bate at the extremity ; second lateral lobe a little longer, but otherwise very 

 similar to the first; third lateral sinus agreeing more or less nearly in size 

 and form with the second ; third lateral lobe about one-fourth longer and 

 somewhat wider than the second, but agreeing nearly in its branches with 

 the same ; fourth lateral sinus of nearly the same size and form as the first. 



The succeeding lobes and sinuses at first diminish abruptly, and then 

 gradually and regularly in size, toward the umbilical margin, the fourth and 

 fifth lobes showing generally much similarity in form to the others, excepting 

 that they are more or less tripartite at the end, with much shorter divisions, 

 while the sixth and seventh are in large examples shortly bifid, and the others 

 merely dentate. The fifth and sometimes the sixth sinuses are merely bilobate 

 at the ends, and the others each terminate in a single broad, transversely- 

 reniform cell, with entirely simple margins. 



I have scarcely a doubt that the shell referred by Binkhorst to Ammo- 

 nites Pierdenalis, von Buch, belongs to this species. One of his figures 

 shows more tendency to develop lateral nodes than is common in the species 

 here described, bul I have seen specimens from Colorado, evidently belong- 

 ing to this species, showing the same character: while the other speci- 

 mens figured by Binkhorst agree well with ours in only showing very 

 obscure, undefined, radiating ridges around the outer half of each side, pro- 

 duced rather by intervening depressions than by the elevation of the ridges 

 themselves above the general surface. 



