MESOZOIC FOSSILS. 625 



in ridge west of south branch head of Gardiner; head of Fawn Creek north- 

 east of Monument Peak, Cinnabar Mountain. 



Pholadomya injsquiplicata n. sp. 



PL LXXIV, fig. 4. 



Of. Pholadomya multilineata Gabb, 1869: Am. Jour. Concbology, Vol. V, p. 10, PI. V, 

 fig. 6. 



Shell small, ventricose, elongate suboval in outline, with prominent 

 approximate beaks situated near the anterior end ; anterior and ventral mar- 

 gins forming a nearly regular curve, which is most prominent a little behind 

 the middle; posterior end rounded, slightly subtruncate above; surface 

 marked by about twenty radiating costa? that vary both in size and in dis- 

 tance from each other and cover the whole valve, excepting a very small 

 space in front and a larger one in the postero-dorsal region. 



Length, 39 mm.; height, 31 mm.; convexity of both valves, 24 mm. 



Pholadomya multilineata, which is associated with P. nevadana, seems to 

 be about as closely related to this species as P. nevadana is to P. Mngi. P. 

 multilineata is larg;er than P. incequiplicata, has more numerous costse (about 

 thirty, according to Gabb), and is more angular at the posterior end, besides 

 differing somewhat in other details of outline. 



Only a few specimens were collected on divide between Fawn Creek 

 and Gallatin Valley, where it is associated with P. Mngi. 



HOMOMYA GALLATINENSIS 11. Sp. 

 PI. LXXIV, figs. 6 and 7. 



Shell of medium size, oblong subcylindrical ; beaks rather prominent, 

 incurved, approximate, and situated near the anterior end of the shell; 

 dorsal margin in front of the beaks declining rapidly to the broadly rounded 

 anterior end, which passes by a gentle curve into the nearly straight dorsal 

 margin. Surface marked by lines of growth and irregular concentric 

 undulations. The posterior end gapes slightly. 



Length, 85 mm. ; height, 42 mm. ; convexity of both valves, 36 mm. 



This species apparently belongs to the subgenus Homomya as defined 

 in Zittel's Handbuch der Palaeontologie, but Fischer does not recognize the 

 group and divides the species that have been referred to it between Arcomya 

 aud Pleuromya. The specimens from Yellowstone National Park do not 



MON XXXII, PT II 40 



