138 EXPLORING EXPEDITION FROM SANTA FE 



Formation and locality. — The horizon at which the teeth of Ptychodus Whipplei 

 occur in the Cretaceous rocks of New Mexico is, according to my observations, very 

 clearly defined. Tt is below the middle of the series, generally within a hundred feet 

 of the yellow Lower Cretaceous leaf-bearing sandstones. 



The rock containing them is a sandy, ferruginous limestone, lying in thin bands 

 divided by la vers of dark calcareous (often bituminous) shale. The associate fossils 

 are Lamna Texana, Gryphsa Piteheri, Ostrea lugubris, 0. uniform}*, Inoceramus proble- 

 maticus, LfragUis, Ammonites percarinatus, A. Macomhii, Scaphites larviformis, &c. 



The Ostrea of which M. Marcou speaks as occurring with Pt. Whipplei is not 0. 

 congesta, as he supposes, but 0. lugubris of Conrad. The place of 0. congesta is a little 

 higher in the series. Gryph&a Pitcheri is found a few feet below. 



Of all these species, specimens were obtained on the banks of the Canadian at 

 Galisteo, at the ford of the Chama, and at the Pagosa. 



Athyeis SUBTILITA, Hall, sp. 



Wherever we found Carboniferous rocks in the AVest, we were sure to meet with 

 this wide-Spread fossil. On the Colorado, west of the San Francisco Mountains, at the 

 junction of the Grand and Green Rivers, in the Sierra la Plata, at Santa Fe,andat Pecos, 

 it is abundant. At all these places, the most common form of the Coal-Measures in tin 1 

 valley of the Mississippi is most numerously represented ; there, as here, varying some- 

 what in size, apparently accordingly as it was well or ill fed. 



In the extreme upper Carboniferous or Permian beds, an Ailnjrls is common which 

 has sometimes been considered as distinct from those obtained below, for it is usually 

 larger and more ventricose than the prevailing type of Coal-Measure specimens. It 

 may possibly be a distinct species, but it is true that in many places in the Coal- 

 Measures A. subtilita is found, assuming precisely the same form and reaching an 

 equal size. It will, therefore, be impossible to make two species of them until some 

 internal characters are found which can serve to distinguish them. 



Sl'lRlFKit CAMEEATUS, Morton. 



This shell is common in the limestone of the Carboniferous series at all points in 

 the route of our expedition where these rocks are exposed. Of the large number col- 

 lected in New Mexico and Utah, and a very much larger number examined, all exhibit 

 nearly the same character, the prevailing type as regards size and markings closely 

 resembling that figured by Professor I lull in Captain Stansbury's Report under the 

 name of S. triplicates. Specimens collected in Eastern Kansas and others obtained 

 from the banks of the Colorado are absolutely undistinguishable. The larger shell, 

 considered by Mr. Davidson identical with S. striata* of Martin, is nowhere met with 

 in New Mexico or Utah. It is abundant in the mountain limestone of Illinois and 

 Missouri, but I have never seen it from the Coal-Measures. Considering the marked 

 difference in form and position of these two shells, it is difficult to resist the conviction 

 that if the European palaeontologists could study what they regard as American 

 representatives of 8. striatus in great numbers as they occur in place, they would con- 

 sider them as forming two distinct species. 



