BRAC.HIOPODA. - 75 



Superfamily Strophomenacea 

 Family Strophomenidae 

 Genus Stropheodonta Hall 



Stropheodonta boonensis Swallow 



Plate 11, figure 11; plate 13, figures 6-12; plate 14, figure 9. 



1860. Strophodonta boonensis Swallow, Trans. St. Louis, Acad. Sci., I, p. 638. 

 1897. Stropheodonta navalis boonensis Schuchert, Bull. U. S. Geol. Surv. 87, p. 425. 

 1918. Stropheodonta boonensis Branson, Geology of Missouri, p. 101, fig. 1. 



Swallow's description — "Shell hemispheroidal. Ventral valve regularly convex, 

 slightly raised along the mesial line; umbo depressed, scarcely modifying the curvature 

 of the valve; beak small, slightly incurved over the area; area straight, narrow, im- 

 pressed, striated vertically, about as long as the width of the shell; ears small, indistinct. 

 Dorsal valve less convex, semielliptical, depressed in front; lateral margins recurved to 

 meet the cardinal border. Surface marked with large rounded, ladiating plications, 

 (which are sometimes angular, irregular and unequal), increased by subdivisions and 

 implantations. 



Length, 1.12; breadth, 1.20; depth of ventral valve, .65." 



Additional description — The hemispheroidal shape is prominent but is often modi- 

 fied by the small ears when they are present, by increased or decreased length of hinge- 

 line, by decrease of convexity of the pedicle valve, and by the greater convexity coming 

 nearer the hingeline than the anterior margin. Average specimens are not slightly raised 

 along the mesial line. In some specimens that grade toward S. inflexa Swallow the 

 umbo modifies the shape of the valve. Measurements of three specimens give the 

 following dimensions: Length 30, 24J^, 22 mm., breadth 37, 35J^, 24 mm., height 

 22, 18, 15 mm. The plications present a considerable amount of variation though 

 they are all coarse; the extremes are an average of 6 to the centimeter and 12 to the 

 centimeter. In plications the specimen with 10 to the centimeter is about like 5. inflexa 

 Swallow but in other respects it is quite distinct. The entire shell is punctate. 



In one direction this species grades toward 5. inflexa Swallow, from which it differs 

 in having coarser plications, length compared to breadth being much less, averaging 

 about 3:4 compared to 9:10 in inflexa and in the umbo not projecting back of the 

 hingeline. In another direction it grades toward 5. callawayeyisis Swallow from which 

 it differs in its greater convexity, shorter hingeline, less prominent ears, and coarser 

 plications. 



Remarks — Schuchert refers 5. boonensis and 5. inflexa to S. navalis boonensis. 

 Swallow's descriptions clearly differentiate these forms and though some specimens 

 intergrade it is an easy matter to refer almost all specimens to their species. 5. navalis 

 is defined by Swallow as having "ears distinctly defined and strongly arched towards 

 the area" while both of these species have the ears poorly defined and not arched toward 

 the area. Both of these species have coarse plications while 5. navalis Swallow is defined 

 as having "numerous plications." 



Occurrence — Snyder Creek shales Callaway and Montgomery Counties. 



Stropheodonta callawayensis Swallow 

 Plate 13, figures 1-5 



1860. Stropheodonta callawayensis Swallow, Trans. Acad. Sci. St. Louis I p. 638. 

 1897. Stropheodonta callawayensis Schuchert, Bull. U. S. Geol. Survey, 87, p. 420. 



Swallow's description — "Shell semi-elliptical, margin regularly curved from the 

 anterior margin to the cardinal extremities. Ventral valve very convex from middle 

 towards the beak; somewhat flattened towards the anterior and lateral margins; the 



