3X8 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



specimens, which some one will doubtless have an opportunity to do here- 

 after. 



This shell is not liable to he confounded with any of the other Upper 

 Missouri species yet known. The form of the volutions of its spire and its 

 surface-markings somewhat resemble those of the last; but its larger size, 

 more numerous whorls, much more produced spire, and proportionally 

 shorter and less expanded body-volution, will ;it once distinguish it. One of 

 its most marked features is the peculiar constricted, screw-like appearance of 

 its spire, produced by the concave upper slope of each whorl, and the con- 

 vexity of the succeeding one above, all the way up. 



The specific name of this shell is given in honor of the late Dr. Hitz, of 

 Washington City, who discovered the only specimen I have yet seen. 



Locality and position. — The type-specimen was brought by Dr. Hitz, 

 along with characteristic fossils of the Fort Benton group, collected at a 

 locality opposite Fort Shea, on the Upper Missouri. 



CEPHALOPODA. 



TETRABEANCHTATA. 



BACULITIDiE. 



The terminology of the parts of the shell in this and the other more or 

 less nearly allied families of the great group of complexly septate Cephalo- 

 poda for which Professor Gill has used the name Ammonitoidea, having 

 undergone some changes since tin 1 publication of most of the works on these 

 types, and some additional changes of this kind being made in the following 

 descriptions, a few words' of explanation on this point may lie desirable here. 



In nearly all of the descriptions of these shells published previous to a 

 comparatively few years back, the siphonal side was called the dorsal, and 

 the opposite the ventral. As pointed out by Van der Hoeven, however, in 

 1849, and more recently by Professor Pictet, Professor Hyatt, and others, 

 the know n posture of the animal of Nautilus in its shell, by analogy, renders 

 it almost certain that in all of the involute, spiral, and arcuate types of the 

 Ammonitoid a, the outer or siphonal side of the curve is really the ventral, 

 and the opposite or inner side the dorsal. Consequently, it becomes equally 

 evident that the siphonal side in the straight Baculite should be also viewed 

 as the ventral, and the opposite as the dorsal surfaces. 



Still, as the opposite arrangement was long believed to have existed, 



