412 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



species vary to a considerable extent. For these reasons. I have placed 



Solenoceras and Diptychoceras doubtfully as synonyms of Ptychoceras. 



The genus Ptychoceras seems to be entirely confined to the Cretaceous 



system. 



Ptychoceras Mo r to n i, M. & H. 



Plate 20, figs. 4, a, b, c. 



Ptychoceras Morloni, Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Philad., IX, 134.— Meek (1864), 

 Soiithsouiau Check-List N. Am. Cret. Fossils, 23. 



Shell small, subcylindrical; larger limb somewhat compressed laterally, 

 round on the outer side, and concave along the inner, for the reception of 

 the closely-appressed smaller limb, which is less compressed laterally, and 

 tapers so very gradually as to appear almost perfectly cylindrical in broken 

 specimens ; surface of both limbs ornamented by rather distinct annular 

 costse, which pass around somewhat obliquely, and become obsolete on the 

 inner side, while, in passing over the ventral or outer side of the larger limb, 

 they sometimes, though rarely, bifurcate, and are each provided with two 

 very small nodes, so arranged as to form two parallel longitudinaal rows; 

 very fine, rather obscure, lines of growth also mark the surface parallel to the 

 costse. 



Septa unusually simple; siphonal lobe oblong, being a little longer than 

 wide, and merely deeply divided into two somewhat spreading, terminal 

 branches, with two short, obtuse, unequal digitations each; first lateral sinus 

 nearly as large as the siphonal lobe, and divided into two short, subequal, 

 smooth, rounded, terminal lobes, that give it a nearly cordate outline ; first 

 lateral lobe a little narrower above, but otherwise very similar in size and 

 form to the siphonal lobe; second lateral sinus agreeing nearly in size and 

 form with the first, excepting that its two short, obtuse divisions show a slight 

 tendency to form each two very short obtuse digitations; second lateral lobe 

 almost exactly like the first in size and form; third sinus scarcely half as 

 large as either of the others and merely faintly bilobate at the end ; anti- 

 siphonal or inner lobe very small, or not more than half as wide, and but 

 little more than half as long, as either of the lateral lobes, and merely tridi- 

 gitate at the end. 



The only specimens of this shell yet known are too imperfect to afford 

 exact measurements. One of them shows a portion of the non-septate limb 

 0.93 inch in length, which measures 0.09 inch in its greater diameter at the 



