BRACHIOPODA. 77 



Swallow's description — "Shell semiellipsoidal, auriculate and costate. Ventral valve 

 very convex, subcarinate, highest near the center; curvature increased regularly from 

 the anterior margin to the beak; flattened towards the lateral margins and sharply 

 rounded along the middle; umbo large, projecting beyond the cardinal line about one- 

 fourth the length of the shell; beak small, strongly recurved; scarcely modifying the area; 

 area straight, narrow, and much impressed; cardinal line as long as the width of the 

 shell. Dorsal valve semielliptical, very concave, with a deep rounded sinus in front to 

 fill the corresponding elevation in the opposite valve; the lateral edges recurved and 

 arched to meet the impressed cardinal border, and form the sinuses leading from the 

 central cavity into the small arched ears. Surface marked by small, rounded, granulated 

 irregular, radiating plications, whose number is increased by subdivisions and implanta- 

 tions. 



Length, 1.45; breadth, 1.11; depth of ventral valve, .94." 



Supplementary description — Shell thin; outline of valves, resting with the margins 

 flat, semi-ellipsoidal in many specimens, in others much constricted toward the hinge- 

 line. Ears small but rather distinctly set off from the rest of the shell by the upturning 

 of the extremities or by a deep groove. Pedicle valve very convex, sharply rounded to 

 subcarinate, highest near the center; curvature increased regularly from the anterior 

 margin to the beak. Umbo large, projecting beyond the hingeline one-third to one- 

 fourth the length of the shell; beak very small, projecting slightly over the cardinal area. 

 Hingeline usually shorter than the width of the shell but occasionally as long. The 

 length of hingeline in 7 average specimens varies from 17 mm. to 23 mm., the mode 

 being 20 mm.; length of hingeline compared to greatest width 3:4 to 4:4; length of shell 

 compared to greatest breadth 10:8 to 10:9. The curvature of the brachial valve is about 

 half to two-thirds that of the pedicle ; at the highest and narrowest part the shell is almost 

 flat; the lateral edges are recurved and arched to meet the cardinal border. Surface 

 marked by small, rounded to subangular, irregular, radiating plications, which increase 

 in number by bifurcation and implantation; 12 to 18 plications to the centimeter. Size 

 given by Swallow about one-tenth above average. Only one of our specimens has a 

 ventral valve depth as great as listed by Swallow. Surface of shell punctate. 



Remarks — The species is readily recognized by the extreme projection back of the 

 hingeline, small plications, and narrow hingeline. No other species of Stropheodonta 

 in the fauna has an average length of hingeline compared to length of shell less than 50 

 per cent greater than this. 



Schuchert uses the specific name "navahs" for this form, but the most characteristic 

 feature, the projecting umbones, is not present in navalis. The ears are not strongly 

 arched towards the area as in navahs and the cardinal line is generally shorter than the 

 width of the shell while it is longer in navalis. 



Occurrence — Snyder Creek shale, Callaway and Montgomery Counties. 



Stropheodonta demissa (Conrad) 

 Plate 11, figures 3 and 8.; plate 14, figures 1-8. 



1842. Strophomena demissa Conrad, Jour. Acad. Nat. Sci., Phila., VIII, p. 258, pi. 14, 



fig. 14. 

 1852. Strophodonta dimosa (?) Owen, Geol. Survey Wisconsin, Iowa and Minnesota, 



tab. 3a, fig. 14. 



1857. Strophomena {Strophodonta) demissa Hall, Tenth Rep. N. Y. State Cab. Nat. 

 Hist., pp. 137-139, figs. 1-2. 



1858. Strophomena demissa Rogers, Geol. Penna., II, p. 827, fig. 666. 



1858. Strophodonta demissa Hall, Geol. Surv. Iowa, I, pt. 2, p. 495, pi. 3, fig. 5. 

 1860. Strophodonta quadrata Swallow, Trans. Acad. Sci., St. Louis, I, p. 639. 

 1867. Strophodonta demissa Hall, Pal. N. Y., IV, pp. 81 and 101, pi. 11, figs. 14-17; 

 pi. 12, figs. 1-5; pi. 17, figs. 2a-2s. 



