CARBONIFEROUS PERIOD. 143 



Camp Apache; Tenney's Ranch, Kaibab Plateau; confluence of . White 

 Mountain and Bkxck Rivers ; Grass Mountain, thirty-five miles north of 

 Pioche ; and foot-hills of Dragoon Mountains, Arizona : Fossil Hill, White 

 Pine County; and Camp Cottonwood, Nevada: fifteen miles sovith of Saint 

 George; near Ophir City ; Rock Canon, Wasatch range, near Provo ; and 

 near Minersville, Utah. 



Spirigera planosulcata Phillips, fip. 



Plate X, fig. 5 a, h, c, and d. 



Spir if er planosulcata V\u\\{\)9,, 1836, GeoIog.v of Yorkshire, ii, 220. 



Terehratula planosulcata Marcou, 1858, Geology of North America, 53. 



Compare Athyris suhlamellosa Hall, 1858, Geology of Iowa, i, pt. ii, 702. 



Compare Athyris crassicardmalis White, ISCO, Jour. Bost. Soc. Nat. Hist., vii, 229. 



Comparo Athyris 2)lanosulcataf Meek and VVortlieu, 1800, Geol. Surv. Illinois, ii, 254. 



Shell rather small, having asubtetrahedral, subpentahedral, or subcircular 

 outline ; both valves more or less gibbous ; greatest breadth a little behind 

 the middle, the valves almost equally capacious. Ventral valve without a 

 proper mesial sinus, but in its place there is usually to be seen a slight mesial 

 attenfling extending from the front to about the middle ; beak small, promi- 

 nent, curving upward so as to bring its small foramen about on a plane with 

 the margin of the valve. Dorsal valve broadly convex, but it is in most 

 cases mesially flattened a little at the front, like the ventral valve ; this 

 slight flattening of both valves produces a little straightening or truncation 

 of the otherwise broadly-rounded front border. 



Surface marked by numerous imbricating lines of growth and occa- 

 sionally by faint traces of radiating striae. 



Length of an average-sized example in tlie collections, twelve milli- 

 meters ; breadth, thirteen millimeters ; height, eight millimeters. 



The characteristics of this little shell as represented in the collections 

 are quite constant, and it seems to agree in all essential respects with the 

 species to which it is here referred ; but, so far as I have been able to make 

 comparisons, I am not without some doubt as to its specific identity with S. 

 planosulcata, nor can I satisfactorily refer it to any other described species. 

 In external characters, which are the only ones that have been observed in 

 the shell under discus-sion, it agrees almost exactly with S. crassicardcnaUs 



