438 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



It should be remembered, however, that the differences here noted are not 

 always so strongly marked as in the septum figured. 



This species seems to be somewhat related to Morton's variety gulosus 

 of S. Conradi, but differs remarkably in its large open umbilicus, showing all 

 of the inner volutions, and in having the last half of its outer volution scarcely 

 deflected at all from the regular curve of the inner turns, or in the slightest 

 degree dilated or straightened along the umbilical side. The sides of the 

 last half of its outer volution are also without nodes, though distinctly costated ; 

 but the costse that extend entirely across to the umbilicus, terminate along 

 its margin in comparatively large node-like prominences. 



Although at one time inclined to think this shell an extreme variety of 

 S. Conradi, a study and comparison of more extensive collections, some years 

 back, satisfied me that it is an entirely distinct species. 



The very slight deviation of the last half of its outer volution from the 

 regular curve of the others, with its large open umbilicus, give it very much 

 the appearance of an Ammonite, to which genus Dr. Owen referred it. He 

 saw only specimens consisting of inner volutions, without the very slightly- 

 deflected half of the last turn, like the specimen represented by our figure 

 3, c; but it is evident that he would have referred even perfect specimens to 

 Ammonites, had he seen them, from the fact that he was in doubt whether 

 even his Scaphites nodosus, in which the characters of the latter genus are 

 much more decidedly marked, might not belong rather to the genus Ammo- 

 nites. Its close relations, however, to S. abi/ssinus, Morton, in which the 

 characteristic deflection of the body-part is sometimes unmistakable, together 

 with the general similarity of its septa to the genus Scaphites, and the 

 scarcely observable tendency of the last half of its outer volution to deviate 

 from the regular curve of the others, are sufficient reasons, I think, for placing 

 it in the latter genus. The existence of such exactly intermediate forms, 

 however, between the Scaphitoid and Ammonhoid types, is an interesting 

 example of the blending together through a comparatively few species of 

 groups that seem strongly distinct, as represented by great numbers of others. 



A single fragment of the outer non-septate portion of this species, repre- 

 sented by figure 3, d, of our plate 35, is accidentally broken in such a man- 

 ner as to expose within, an exceedingly interesting specimen of one of those 

 curious bodies for which Parkinson in 1811 proposed a generic name Trigo- 

 nellites, afterward (in 1831) named Aplychus by Meyer. This specimen was 



