PROCEEDINGS OF THE NATIONAL MUSEUM vol.78 



DESCRIPTION OF SPECIES 



Family PTERIIDAE Meek 

 Genus PTERIA Scopoli 



Subgenus Oxytoma Meek 



PTERIA (OXYTOMA) species undetermined 



Plate 1, Figure 5 



A single small external mold of a species of Pteria {Oxytoma)^ 

 jjreserved in the living chamber of Acompsoceras americanwn^ pre- 

 sents features that mark it as an unnamed form. In the absence of 

 other material it is not possible to tell whether this single individual 

 is juvenile or mature, and the writers have therefore left it unnamed. 



The specimen represents a left valve with a width of 3 millimeters 

 parallel to the hinge margin and a height of 2 millimeters. The 

 posterior wing is proportionately large and the body of the shell 

 stout and well rounded. The ribs, about 30 in number, are of even 

 size and rather closely spaced on the anterior and posterior parts of 

 the valve, but are more distant and alternate in height on the middle 

 section. The posterior wing is ribbed, relatively large, and shallowly 

 emarginate. 



No comparable species has been recorded from beds in the Interior 

 Province as old as the Aspen shale except a large undescribed indi- 

 vidual from the Thermopolis shale of the Crow Indian Reservation 

 in southern Montana. Pteria {Oxytovia) nehrascana (Evans and 

 Shumard)" and P. {0.9) haydeni Hall and Meek ^* are both much 

 larger and come from much higher horizons. 



Cat. No. 73763, U.S.N.M. 



Family OSTREIDAE Lamarck 

 Genus OSTREA Linnaeus 



OSTREA species 



Plate 2, Figure 3 



Several of the specimens of Epengonoceras aspenanum were over- 

 grown by a small oyster with subcircular valves, almost unsculptured 

 and lacking in features which would serve to differentiate it from 

 similar simple forms occurring at many horizons. It is recorded 

 here for the sake of completeness. These specimens might be the 



^ Meek, P. B., Invertebrate Cretaceous and Tertiary fossils of the Upper Missouri 

 country : U. S. Geol. Survey Terr. Kept., vol. 9, p. 34, pi. 16, fig. 3 ; pi. 28, fig. 11, 1876. 

 " Idem, p. 33, pi. 16, fig. 2. 



