100 BRACHIOPODS. 



beak; while on young or compressed individuals it is faintly 

 marked, even anteriorly; lateral slopes each occupied by four 

 to seven or eight simple angular plications. 



Yentral valve with its beak abruptly pointed and very 

 strongly incurved upon that of the other valve in adult shells,, 

 but less distinctly curved, and showing a small opening under 

 its apex in young examples ; mesial sinus deep and well defined 

 in gibbous specimens, and less so in the young or more com- 

 pressed forms, never quite reaching the point of the beak, and 

 always having three simple, rather angular plications in the 

 bottom that extend like the others to the apex of the beak in 

 well-preserved specimens ; lateral slopes each occupied by 

 from five to seven simple plications. Entire surface of both 

 valves marked by numerous very regular, strongly zig-zag, 

 prominent, sublaminar marks of growth that become nearly or 

 quite obsolete, sometimes, on old examples. Length of a me- 

 dium-sized, moderately gibbous individual, 0.75 inch ; breadth, 

 0.81 inch; convexity, 0.66 inch. ( Meek.) 



Horizon and localities Lower Silurian, Hudson shales : 

 Louisiana, Cape Girardeau. 



Rhynchonella dentata (BALL). 



Plate xli, fig. 3. 



Atrypa dentata Hall, 1847 : Pal. New York, vol. I, p. 148, pi. xxxiii, figs. 



]4a-c. 

 Rhynchonella dentata Hall, 1859 : State Cab. N. Y., 12th Ann. Rep , p. 65. 



Smaller and more slender than R. eapax. 

 Horizon and localities. Silurian, Hudson shales : Cape 

 Girardeau. 



Rhynchonella missouriensis SHUMARD. 



Rhynchonella missouriensis Shumard, 1855: Geol. Sur. Missouri, Ann. Rep., 



p. 204, pi. C, figs. 5a-e. 

 Rhynchonella missouriensis Meek & Worthen, 1868: Geol. Sur. Illinois, 



vol. II, p. 153, pi. xiv, figs. 4a-b. 



Shell gibbous, subtriangular, beaks sharp ; greatest width 

 usually near the front, but very variable in different ages of 

 the shell. Vertical valve much more elevated than the dorsal 



