76 
Locality and Horizon. Lower Mississippian of Colorado. In the 
Minnewanka region in the Mississippian (whence the specific name) of 
sections 2-22 (C), 23 (c), 24 (r), 25 (R), 26 (R), 27 (R). 
Myalina wyomingensis (Lea) 
1853. Modiola wyomingensis Lea, Acad. Nat. Sci., Phil., Jour. (2), vol. 2, 
p. 205, PI. 20, fig. 1 a; Claypole, 1886, Proc. and Coll. Wyoming 
Hist, and Geol. Soc., vol. 2, pt. 2, p. 247; Myalina wyomingensis 
Girty, 1903, TJ.S. Geol. Surv., Prof. Paper 16, p. 422, PI. 8, figs. 8-13. 
Remarks. This species is distinguished by the straight umbonal 
ridge which meets the hinge-line at an angle of 45 degrees and by the 
small anterior lobe, the beaks being thus not quite terminal. In size and 
general outline it is quite closely similar to M. swallovi McChesney. In 
that species, however, the umbonal ridge is less defined and not so straight, 
and the posterior margin meets the hinge-line in a curve. The surface, 
moreover, is smooth or marked only by fine, concentric lines. Our speci- 
mens are somewhat smaller than those described by Girty from Colorado. 
Locality and Horizon. In the Pennsylvanian of Pennsylvania and 
Colorado. In the Minnewanka region in the Permian of section 1-14 (c). 
Genus, Deltopecten Etheridge 
Deltopeden ocddentalis latiformis n. var. 
Plate VII, figures 11 a, b; 12 
A bibliography of the species is given (1909) by Girty 1 ; another is 
listed (1915) by Mather. 2 
Description . Shell flabelliform, with transverse diameter (from 
anterior to posterior border) slightly greater than the distance from the 
hinge-line to the basal border; in consequence, the angle formed at the 
beak between the anterior and the posterior umbonal slopes is broad, 
slightly exceeding a right angle. Hinge-line nearly or quite equalling 
the greatest length of the valves. Length, the diameter from the anterior 
to the posterior margin, 30 mm. ; height, from hinge-line to basal margin, 
23 mm.; greatest thickness of the conjoined valves 8 mm. Angle of 
divergence of the umbonal slopes at the beaks 100 degrees (in one specimen 
90 degrees). 
Left valve sloping broadly and flatly in all directions from the slightly 
convex umbonal region. Ears nearly equal in size, the anterior the more 
obtuse and defined by a deeper sinus from the body of the shell; it is marked 
by six or seven radiating ribs which are finely crenulated by concentric 
striae. Posterior ear separated from the body of the valve by a broad 
and shallow depression; on this ear the concentric markings are the more 
conspicuous and the radiating striae are fine and numerous — 18 or 20 in 
number. Surface of valve marked by strong radiating costae between 
which are from one to three intercalated weaker striae; only the stronger 
costae are present at the beak, whereas at the ventral margin there are, 
in a space of 6 mm., about ten striae, of which four are strong. 
» U.S. Geol. Surv., Bull. 389, p. 84. 
J BulI. Sci. Lab. Denison Univ., vol. 18, p. 227. 
