194 THE DEVONIAN OF MISSOURI. 



Uncinulus mutabilis Clarke, 1908, N. Y. State Mus., Mem. 9, pt. I, p. 171, pi. 28, 



figs. 3-8. 



Description — "Shell varying from ovate to spherical. Ventral valve sometimes 

 depressed, generally most convex in the umbonial region; beak small, pointed, closely 

 incurved over that of the opposite valve. Dorsal valve gibbous; beak incurved beyond 

 the hinge line; cardinal border on each side of the beak, concave. Surface marked by 

 twenty to twenty-six depressed rounded simple plications, of which about six or eight 

 are slightly raised towards the front of the dorsal valve into an indistinct mesial eleva- 

 tion; and five or six depressed near the front of the ventral valve, and extended into 

 a short linguiform prolongation; concentrically marked by fine undulating striae. 



" [Remarks] — The plications on this shell are usually simple, though in a few speci- 

 mens one or two of them are seen to bifurcate * * * the plications in front 

 and at the sides are marked with a central impressed line towards the margins of the 

 valves. 



"In form and general aspect this species varies greatly, being sometimes longitudi- 

 nally ovate or oblong, in others globose and subpentagonal. The extremes of these 

 varieties, without the intermediate forms, would appear to present well-marked specific 

 differences; but a careful study of the series shows such an imperceptible gradation of 

 form as to leave no doubt of their identity. 



"The cast of the ventral valve shows a strongly marked ovate muscular imprint, 

 the details of which, and of the accessory parts, vary in individuals of different forms 

 and proportions." — Hall, 1859. 



Remarks — The observed specimens have from about 23 to about 38 plications on 

 each valve, from 3 to 6 plications on the fold, and from 2 to 5 plications in the sinus. 

 The surface of the valves is marked with concentric lines. 



Uncinulus subpyramidatus n. sp. 

 Plate 49, figures 23-34 



Description — Shell small, globose-subpyramidal in form, transversely subelliptical 

 in outline. The dimensions of a complete example are: Length 9.7 mm., width 11.2 

 mm., thickness 8.9 mm., width of sinus 4.7 mm. 



Pedicle valve depressed convex in the umbonal region, the surface a little inflected 

 to the cardinal margin, curving to the antero-lateral margins with an increasing con- 

 vexity as it approaches the margin; mesial sinus originating anterior to the beak, be- 

 coming moderately deep anteriorly and produced in front in a long lingual extension 

 lying in nearly a right angle to the plane of the valve; the beak small, incurved over 

 that of the brachial valve. Surface of the valve marked by simple, rounded plication, 

 which become faint towards the beak, about three to about eight occupy each lateral 

 slope of the valve, and one or two the mesial sinus, grooved longitudinally along their 

 median line as they approach the anterior margin. 



Brachial valve depressed convex in the umbonal region, the surface curving with a 

 strong convexity as it approaches the antero-lateral margins, the mesial fold originating 

 at, or anterior to, the beak, becoming moderately elevated in front, and curving rather 

 abruptly to the anterior margin. Surface marked by plications similar to those of the 

 pedicle valve, about three to about eight on each lateral slope, and two or three upon 

 the mesial fold, grooved longitudinaly. 



Surface of both valves marked by concentric lines of growth. 



Remarks — This little shell is less globular and more coarsely plicated than » U. 

 globulus. 



It differs externally from the associated Wilsonia wadei in its less angular and 

 more globose form, the margins of the valves not being abruptly deflected. 



