396 DESCRIPTIONS OF NEW SPECIES OF FOSSILS, 



AMMONITES PERCARINATUS, n. sp. 

 PLATE IV. FIG. 2, a-c. 



Discoidal, depressed ; umbilicus wide and shallow ; volutions about four or five, all 

 visible in the umbilicus, scarcely one fourth of each embraced in the succeeding one ; 

 shell thin ; surface marked by thirty-eight to forty-five prominent flexuous sharp ribs 

 some of which originate in the umbilicus, and others upon the latero-ventral margin, 

 and all extend to the dorso-lateral edge, where they bend abruptly forward, and ter- 

 minate before reaching the dorsal line, which is marked by a thin sharp carina extend- 

 ing to the aperture. Ribs thickened and sometimes nodose towards the periphery. 



Our specimens are all casts of the interior with fragments of the shell adhering, and 

 the condition is such as to give no means of determining the character of the septa. 

 Among American species, it resembles in general appearance the A. dbyssinius of 

 Morton, (Jour. Acad. Nat. Sci., Vol. VIII. p. 209, PI. X. fig. 4,) from which it may be 

 at once distinguished by its wider umbilicus and dorsal carina. The same remarks 

 would apply to a comparison of this species with the A. splendens of Sowerby, in its 

 young state, as given by D'Orbigny (Terrains Cretaces, p. 222, PI. 63, fig. 3). It 

 differs also from the A. helius, D'Orbigny, (loc. cit.,p. 187, PI. 57, figs. I and 2,) in its 

 much wider umbilicus and more sharply elevated carina, while in the European species 

 the annulations all reach the umbilicus. In the proportions of the umbilicus it resem- 

 bles the A. heliacus and A. angulicostatus, D'Orbigny, (loc. cit., PI. 25 and 46,) but 

 differs in its sharp dorsal carina, as well as in other characteristics. In external char- 

 acters this species bears a close resemblance to A. aalensis, Zeit. (A. candicans), 

 D'Orbigny, (loc. cit, p. 238, PI. 63,) from the upper Lias. 



Locality and Position. Five miles below the mouth of Vermilion Eiver on the 

 Missouri ; in division No. 2 of Section. 



HAMITES MORTONI, n. sp. 



* 



PLATE IV. FIG. 3, a-c. 



Cylindrical, curved, increasing very gradually in diameter towards the larger ex- 

 tremity ; surface crossed obliquely by sharp annulations, which are less strong upon 

 the ventral side, and sharper and stronger upon the dorsal side. A few of these 

 annulations are nodose on the back, and some of them also bifurcate and again unite 

 after making half a revolution. Annulations narrower than the space between them. 

 Dorsal lobe shorter than the superior lateral lobe, bifurcate (the two sides a little 



