230 THE DEVONIAN OF MISSOURI. 



Brachial valve sub-convex to flat, characterized by a mesial sinus which is narrow 

 and deep near the beak, and broad and shallow toward the anterior margin. Cardinal 

 margins make a more obtuse angle with the beak than in the opposite valve. Beak 

 small and inconspicuous. A cast of the interior shows two elongate and narrow muscle 

 scars separated by a deep groove. 



Surface of both valves marked by radiating costae of uniform size, which bifurcate 

 frequently; 4 occur in the space of 1 mm at a point midway between the beak and 

 anterior margin. These are crossed by coarser concentric lines of growth. 



Remarks — This species resembles Dalmanella subcarinata Hall of the Lower Helder- 

 bergian beds, but is smaller, with the pedicle valve more highly convex and more elon- 

 gated, and the cardinal margins meeting the beaks at a sharper angle. 



Horizon — Lower and upper beds. 



Holotypt and Paratypes No.^ 27482 Walker Museum. 



Family Rhipidomellidae 

 Genus Rhipidomella Oehlert 



Rhipidomella emarginata (Hall) 



Plate 61, figures 3-5 



1859. Orlhis oblata var. emarginata Hall, Nat. Hist. N. Y. Pal., vol. 3, p. 164, pi. 10a, 



figs. 4-6, 1861. 

 1913. Rhipidomella emarginata Maynard, Geol. Surv., Md. Low. Dev., p. 302, pi. 55, 

 figs. 1-8. 



Description — Shell small, a little broader than long, the hinge line less than the 

 greatest width, from subtriangular to subcircular in outline. The dimensions of an 

 average small individual are: Length 10 mm., width just below the middle of the shell 

 11 mm. The brachial valve of a larger form measures 13 mm. in length and 14 mm. in 

 width. 



Pedicle valve convex, the greatest convexity posterior to the middle, with a mesial 

 sinus which is ill-defined near the beak, but becoming broad and more conspicuous 

 toward the anterior margin. Hinge line straight, cardinal margins sloping abruptly 

 from the beak, extremities rounded, giving the shell a circular outline. 



Brachial valve more convex than the opposite, without well-defined fold; sur- 

 face sloping uniformly from the middle to the anterior and lateral margins. Beak 

 inconspicuous, slightly incurved below that of the opposite valve. 



Surface of both valves covered with simple, radiating costae near the anterior 

 margin, 3 occur in the space of 1 mm. These are crossed by coarser concentric growth 

 lines, which are most prominent close to the anterior margin. 



|^t 13 Remarks — This Helderbergian species is represented in the collection by one entire 

 specimen, and a few separate valves. These are smaller than the forms figured from 

 the Lower Helderbergian beds of New York, and the beak is less elevated. 



Horizon — Lower and upper beds. 



Figured specimens No. 27480 Walker Museum. 



Rhipidomella musculosa Hall 



Plate 61, figures 6-8. 



1857. Orthts musculosa Hall, Tenth Ann. Rept. N. Y. State Cab. Nat. Hist., "p. 46. 

 1859. Orthis musculosa Hall, Nat. Hist. N. Y. Pal., vol. 3, p. 409, pi. 91, figs. 1-3; 



pi. 95, figs. 1-7, 1861. 

 1892. Rhipidomella musculosa Hall and Clarke, ibidem, vol. 8, pt. I, pp. 190, 210, 225, 



pi. 6a, fig. 5. 



