40 PALEONTOLOGY. 



moderately and evenly convex ; beak very small, or little distinct from the 

 cardinal margin, and somewhat incurved ; area merely linear ; mesial fold 

 much depressed, being scarcely so defined as to correspond to the sinus of 

 the other valve in size, and becoming nearly or quite obsolete before reach- 

 ing the beak. Surface of each valve marked by about thirty to forty small, 

 generally simple, radiating costae, or strise, about eight to ten of which 

 occupy the mesial sinus, and about as many the mesial fold, where they 

 sometimes bifurcate. 



Length of a medium-sized specimen, measuring from the umbo of the 

 ventral valve to the front, 0.57 inch ; from the umbo of dorsal valve to 

 the front, 0.49 inch; convexity of the two valves, 0.40 inch; breadth of 

 a large specimen, 0.90 inch ; length of ventral valve from beak to front, 

 0.77 inch; height of area, 0.38 inch. 



This species is related to S. Archiaci of Murchison, from the Upper 

 Devonian rocks of Russia, but differs in having the dorsal valve less convex 

 and its mesial fold more depressed, while the foramen of its ventral valve 

 is proportionally much narrower in all of our specimens. I have likewise 

 been unable to see any traces of the fine surface-granulations observed on 

 that species, though the specimens are scarcely in a condition to have pre- 

 served such fine surface-markings, if they ever existed. In general form, 

 as well as in its high, large area, it has more the aspect of a Cyrtia or 

 Cyrtina than of a Trig onotr eta ; but as none of the specimens show any 

 indications of the foramen being closed by a false deltidium, or of a punctate 

 structure, I have preferred to refer it provisionally to the typical section 

 of the genus Spirifer. 



Professor Hall and Mr. Whitfield have described, in the Twenty-tliird 

 Report of the Regents on the New York State Cabinet, Natural History, 

 238, pi. 11, figs. 21-24, a species very similar to this, from the Hamilton 

 Group of Iowa. Their figures represent their shell as having a somewhat 

 higher and slightly less arch area, as well as a proportionally wider foramen; 

 but, in nearly every other character, it certainly agrees very closely with the 

 species here described. 



Locality and position. — Longitude 115° 26' W. ; latitude 39° 30' N., 

 from a dark limestone of Devonian age; Colonel Simpson's collection. 



