132 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



costae that are somewhat stronger on adults. Other species are not 

 sufficiently similar to warrant detailed comparison. 



Discussion. — The specimen illustrated by Walcott ( 1884) is smaller 

 than the largest specimens of the species, and therefore its costae are 

 poorly developed. Larger specimens in the U.S. National Museum 

 collections from the same locality have somewhat stronger and longer 

 costae, and some have three instead of two on the fold. Smaller speci- 

 mens in the collection, on the other hand, have weaker costae ; some 

 lack costae because these do not form until the shell has attained a 

 length near 6 mm. 



Convexity increases with length, as in other species of the genus. 

 Therefore, along with the late-forming costae, this feature combines 

 to produce what appears to be a rather variable species. The variation 

 is not great, however, when growth factors are considered. Small and 

 medium size specimens are rather flat and may be smooth; larger 

 specimens are convex and may have as many as three costae on the 

 fold and on each flank. 



This species is important as the earliest known species in which the 

 stolidium appeared, although in very rudimentary form. This species 

 may be related to Stenoscisma, with its strongly trigonal outline, rela- 

 tively well-developed stolidium, and camarophorium that began as 

 a trough in the extreme apex rather than as a knoblike swelling at the 

 top of the septum as in early genera. 



Occurrence and abundance. — Mississippian limestone in canyon 

 directly south of Conical Hill, on east side of Secret-Canyon-Road 

 Canyon, Eureka District, Nev. (Walcott Loc. 651) (18 specimens 

 USNM 14344) ; Diamond Peak Formation, at Conical Hill, Windfall 

 Canyon, SW.14 sec. 36, T-19-N, R-53-E, 3 miles south of Eureka, 

 Nev. (10 specimens USNM 142485) ; Diamond Peak Formation, top 

 of Conical Hill, east side of Windfall Canyon road, 3 miles east-south- 

 east of Eureka, Nev. (4 specimens USNM 142486). All of these are 

 from essentially the same locality ; one in the lot from loc. 65 1 is the 

 holotype, the rest are paratypes ; those listed separately are topotypes. 



Age. — Late Mississippian (probably Chester). 



COLEDIUM TORVUM Grant n. sp. 



Plate 13, figs. 1-2 ; figure 31 



Shell medium to large for genus, strongly biconvex ; outline ovate, 

 slightly elongate in youth, slightly transverse as adult; commissure 

 uniplicate; fold moderately high at anterior commissure, broad, with 

 gently rounded or slightly flattened crest, but standing only slightly 

 above flanks of valve anterior to about 5 mm. from brachial beak; 

 sulcus shallow, beginning about 7 mm. anterior to pedicle beak, de- 



