108 THE DEVONIAN OF MISSOURI. 



smaller than average C. hamiltonensis but some are large. Many of the small speci- 

 mens have only 5 plications on each side of the fold and sinus. 



Occurrence — Mineola of Callaway, Montgomery and Warren Counties. 



Cyrtina missouriensis (Swallow) 

 Plate 16, figure 9; plate 19, figures 1-4 

 1860. Cyrtia Missouriensis Swallow, Trans. St. Louis Acad. Sci., I, pp. 647-648. 

 1889. Cyrtina missouriensis Miller, North American Geol. Pal., p. 343. 

 1894. Cyrtina umbonata Keyes, Missouri Geol. Surv., V, p. 90. 

 1897. Cyrtina missouriensis Schuchert, Bull. U. S. Geol. Surv. 87, p. 198. 

 1909. Cyrtina missouriensis Greger, American Jour. Sci., XXVII, p. 378. 



Swallow's description — "Shell small, angular, beak and cardinal extremities salient. 

 Ventral valve very prominent, regularly arched from the beak to the front and anterior 

 portions of the lateral margins; beak pointed and incurved; sinus well defined, becoming 

 deep and broad in front where it projects into the fold of the dorsal valve; area somewhat 

 regularly concave, triangular; the hinge line but little longer than the sides, and about 

 equal to the greatest width of the shell; foramen rather large, widening regularly from 

 the beak to the hinge-line, closed by a convex pseudo-deltidium, which is perforated 

 above the middle. Dorsal valve semi-elliptical, convex, flattened towards the cardinal 

 extremities, which are somewhat produced; mesial fold convex, well defined and bounded 

 by grooves larger and deeper than those separating the plication; beak incurved against 

 the lower extremity of the deltidium. Surface marked with rounded plications, seven 

 to nine on each side of the mesial fold, and eight to ten on each side of the mesial sinus. 

 There is also one plication, somewhat smaller than the others, on each slope of the 

 mesial fold and sinus. The plications bounding the mesial sinus are larger than the 

 others; the internal casts of the shell show all the plications less distinctly. The shell 

 is also marked with fine imbricating concentric lamellae and numerous small punctures. 



This species is more nearly allied to the Cyrtia umbonata of Prof. Hall than to any 

 other. It differs, however, from that in having plications on the mesial fold and in the 

 sinus of the ventral valve; the plications are more numerous and extend to the beak; 

 the foramen is wider at the base." 



Supplementary description — Shell usually small but some as large as large specimens 

 of C. hamiltonensis Hall. Pedicle valve regularly arched or straight from the beak to 

 the front. Beak pointed and straight or incurved. Area plane or concave. Delthyrium 

 of about the same proportions as in C. hamiltonensis Hall, wider near the hinge line in 

 some specimens. Brachial valve moderately convex to almost flat; mesial fold usually 

 well-defined but in some specimens indistinct. Surface marked by 7 to 11 rounded 

 plications on each side of the fold and sinus. The plications bounding the fold and sinus 

 are usually slightly larger than those on the sides of the shell. 



Remarks — Swallow's description seems to have been based on a selected group of 

 specimens that omitted the variations. The supplementary description is based on speci- 

 mens from the type region. The species differs from C. hamiltonensis Hall in the presence 

 of plications on the fold and sinus; in the fold and sinus being less distinct; and in the 

 plications being more numerous and uniform. 



Occurrence — -Abundant in the Callaway of Callaway, Montgomery and Warren 

 Counties. 



Cyrtina triquetra (Hall) 



Plate 19, figures 9-12 



1858. Cyrtia triquetra Hall, Geol. Survey Iowa, I, pt. 2, pp. 513-514. 

 1868. Cyrtina triquetra Meek, Trans. Chicago Acad. Sci., I, p. 99. 

 1868. Cyrtina triquetra Meek and Worthen, Geol. Survey Illinois, III, p. 436, pi. 13, 

 fig. 4. 



