76 THE DEVONIAN OF MISSOURI. 



convex or raised portion broadly ovate; ears large, well defined, and arched towards 

 the cardinal bor-dei ; extremities recurved towards the center of cne valve; area common 

 to both valves, wider in the middle, as long or shorter than the width of the shell; 

 umbo large, prominent. Dorsal valve concave following the curvatures of the opposite 

 valve; lateral portions, towards the cardinal line, curved down and arched to form the 

 concave surface of the ears; cardinal border crenulated. Surface marked with numerous 

 small, rounded, granulated radiating costae, and by fine concentric striae and lamellar 

 imbricating bands, which are most abundant near the margins. 



"Length 1.30; breadth 1.35; depty of ventral valve .52. 



"This fossil most resembles S. detmssa of Conrad; but it is more convex, its ears 

 are better defined and less prolonged on the cardinal line. It may be distinguished 

 from 5. aequicostala Swallow by its larger and better defined ears, and by the ovate 

 form of the convex portion of the ventral valve." 



Revised description — Shell semicircular to semielliptical, margin regularly curved 

 to near the cardinal extremities where, in most cases, it turns sharply outward. Pedicle 

 valve moderately convex with the greatest convexity in or posterior to the middle; the 

 convex portion sloping rather uniformly toward the sides but interrupted at the base 

 of the ears; ears large, acute, arched toward the cardinal border; umbo of moderate to 

 small size. Brachial valve concave, following closely the curvature of the opposite 

 valve, curving downward near the cardinal line to form the concave surface of the ears. 

 Surface of both valves marked with moderately large plications which increase by im- 

 plantation and occasionally by bifurcation, generally 8 to 10 plications to the centi- 

 meter, occasionally 12 or 6. Some specimens grade towards 5. boonensis Swallow, 

 differing in the smallness of the umbo, greater length of the hingeline, and much more 

 numerous plications. 



Three average specimens measure, width 47mm., 58 mm., 46 mm.; length 35 mm., 

 32 mm., 31 mm.; height 16 mm., 15 mm., 15 mm. The width of the cardinal area in 

 six average specimens is 2}4 to 314 mm., in an unusually thick shell the area is 6 mm. 

 wide. 



The species grades toward 5. demissa (Conrad) from which it differs in its greater 

 convexity, greater length of hingeline, coarser plications, thicker shell, wider cardinal 

 area, and greater width compared to length. It also grades toward S. boonensis Swallow 

 from which it differs in lesser convexity, longer hingeline, greater width compared to 

 length, and less prominent umbo. 



Remarks — Swallow's description of 5. callawayensis does not agree in several respects 

 with the specimens which are here referred to that species. The greatest discrepancies 

 are in the extremities of cardinal border usually not being recurved towards the middle 

 of the valve, the umbo not being large, and the plications being of medium to large size. 



As this is an abundant form in the fauna, it seems unlikely that Swallow would not 

 have described it with the rest and the description of 5. callawayensis is the only one 

 that agrees in even a general way. For these reasons our specimens are referred to this 

 species and the description modified to agree. Specimens may be selected that would 

 fit Swallow's description but they are rare in the fauna and seem to be merely variations 

 from the general type. 



Occurrence — Snyder Creek shale, Callaway and Montgomery Counties. 



Stropheodonta cymbiformis Swallow 

 Plate 12, figures 1, 2, 3. 



1860. Strophodonta cymbiformis Swallow, Trans. St. Louis, Acad. Sci., I, pp. 635-636. 

 1860. Strophodonta subcymbiformis Swallow, Ibid., p. 636. 

 1860. Strophodonta kemperi Swallow, Ibid., pp. 636-637. 

 1860. Strophodonta altidorsata Swallow, Ibid., pp. 637-638. 



1897. Stropheodonta navalis, cymbiformis, subcymbiformis and altidorsata Schuchert, 

 Bull. U. S. Geol. Surv., 87, pp. 424-425. 



