26 



PALAEONTOLOGY OP THE UPPER MISSOURI. 



Pander), is a true Streptorliynchus of King. Now, as these two names of Pander's 

 clearly antedate those proposed by D'Orbigny and Prof. King, and there can be no 

 doubt in regard to the types of Pander's groups, the law of priority compels us to 

 adopt his names. The fact that he did not clearly define his genera, and proposed 

 many species based upon mere varieties or individual modifications of a few species, 

 is no reason for setting aside his generic names, when his figures and descriptions 

 leave no doubts in regard to the genera to which his types belong. 



The genus Hemipronites was first introduced during the deposition of the Silurian 

 rocks ; H. deformis (= Orthis deformis, Hall, Pal. N. Y., Vol. Ill, p. 174, pi. xiii, 3 

 a, b) and H. Woolworthanus (= Strophomena Woolworthana, Hall, ib., p. 192), from 

 the Lower Helderburg rocks of N. Y., being Silurian examples of this group. 

 The genus also occurs in the Devonian, Carboniferous and Permian rocks, probably 

 attaining its maximum development in the Carboniferous. It seems to have become 

 extinct before the commencement of the Triassic period, as we have no well 

 authenticated knowledge of its existence in beds of that age. 



Hemipronites arctistriatus, H. alternatus (= Orthisina arctistriata, and 0. alter- 

 nata, Hall, Thirteenth An. Kept. Eegents University, N. Y., Dec. 1860, p. 80 and 

 81) 1 and H. proximus (== Hipparionyx proximus, Vanuxem, Rept. 3d Geol. Dist. 

 N. Y., 1842, p. 124) are American examples of this genus from Devonian deposits. 



Hemipronites crassus. 



(PLATE I, Fig. 7, O, 6, C, d.) 



Orthisina crassa, MEEK & HAYDEN, Deo. 1858, Proceed. Acad. Nat. Sci. Phila. 260. 



Shell of medium size, snbquadrate, rather compressed, becoming thickened with age ; hinge nearly or quite equal- 

 ling the greatest breadth of the shell ; front broadly rounded ; lateral margins meeting the hinge nearly at right 

 angles, sometimes slightly sinuous near the hinge. Surface ornamented by numerous straight radiating striae, 

 which number near the beaks some thirty or forty to each valve, but increase by the implantation of others between, 

 so as to form one hundred to one hundred and twenty-four around the margin ; crossing these striae are numerous 

 fine elevated concentric lines, which are not only quite distinct in the depressions, but on well preserved specimens 

 are prominent upon the strise, to which they impart a creuulated appearance, as seen by the aid of a lens. Adult 

 specimens generally have also several strong concentric imbricating marks of growth. 



Larger or ventral valve nearly flat ; cardinal edge sloping a little to the lateral margins ; beak somewhat promi- 

 nent, and often distorted, or slightly twisted to one side ; area flat, rather broad, and usually inclined backwards 

 over the hinge angular along its margins ; pseudo-deltidium thick, prominent, nearly or quite closing the foramen ; 

 cardinal teeth not very prominent ; scars of the adductor muscles large, separated by a sharp, rather prominent 

 mesial ridge, and in old shells deep and well defined. 



Dorsal valve moderately convex in the middle, and flat or concave on each side of the slightly convex umbo ; 

 cardinal process rather narrow. Interior of both valves marked by radiating striae around the borders. 



Length of a rather large specimen, 1.25 inch ; breadth, 1.30 Inch ; convexity of the two valves, 0.46 inch. 



At the time we first described this as a new species we had not seen accurate 

 figures of several analogous European forms now regarded as varieties of H. crenistria 

 (= Spirifer crenistria of Phillips). Since seeing Mr. Davidson's excellent figures 

 and descriptions of the various forms now included by him under that name, we are 

 led to doubt whether our shell is entitled to rank as a distinct species. Indeed, 

 supposing H. crenistria to vary to the extent admitted, it would seem to be im- 

 possible to assign any very definite limits to such a protean species, and hence it 



1 Prof. Hall subsequently, in a foot-note on p. 112 of the same Report, refers these species to 

 Streptorhynchus. 



