250 THE DEVONIAN OF MISSOURI. 



Meristella carinata sp. nov. 

 Plate 66, figures 12-21 



Description — Shell of medium size, sub-quadrate in outline, relative length and 

 width variable, sometimes equal. The dimensions of a perfect individual, the holotype, 

 are: Length 23 mm., width 24 mm. Hinge line less than the greatest width of the 

 shell. Cardinal margins gently concave, forming depressed oval areas on both sides of 

 the beak; extremities rounded. 



Pedicle valve sub-quadrate in outline, arched from beak to base, depressed convex 

 to about the middle, and then becoming somewhat concave to form a broad, shallow 

 sinus, which at the anterior margin is projected into an acute linguiform extension. 

 Beak elevated, rounded, and closely incurved over the brachial beak. An impression 

 of the interior shows the large and strongly defined, fan-shaped muscular impressions. 



Brachial valve strongly convex to gibbous, the median portion elevated into a 

 sharply sub-carinated or rounded fold, which in some specimens is especially well 

 defined at the anterior margin. From the fold the sides slope steeply to the lateral 

 margins. Beak incurved below that of the opposite valve. The interior shows the im- 

 pression of a narrow median septum extending over half way to the base. 



Surface of both valves marked by concentric lines of growth, which are paralleled 

 by numerous finer striations. 



Remarks — This is a common species in the collection, and is represented by entire 

 specimens and by individual valves. It approaches Meristella lata Hall, of the New York 

 Oriskany, but differs in its smaller size, in the more sharply defined median portion of 

 the brachial valve, in the more depressed pedicle valve, and in the projection of the 

 sinus into linguiform extension. This form is especially characterized by the carinated 

 brachial valve, and this feature distinguishes it from associated species. 



Horizon — Lower beds. 



Holotype and Paratypes No. 27492, Walker Museum. 



Meristella elliptica sp. nov. 

 Plate 66, figures 4, 5 



Description — Shell of medium size, elongate elliptical in outline, length considerably 

 greater than the width. The dimensions of two perfect pedicle valves are: Length 28 

 mm., width 22 mm., and length 23 mm., width 19 mm., the maximum width midway 

 between the beak and anterior margin. 



Pedicle valve moderately convex, no indication of sinus, but a slight flattening of 

 the shell is visible toward the rounded anterior margin. Beak prominent, incurved, 

 apparently acute. Surface marked by fine concentric growth lines which are especially 

 prominent near the base. 



Brachial valve unknown. 



Remarks — The species approaches Meristella laevis Hall, but differs in its more 

 elongate form, and in the absence of a sinus. 



Horizon — Lower beds. 



Holotype and Paratypes No. 27490, Walker Museum. 



Meristella ovalis sp. nov. 



Plate 66, figures 6-8 



Description — Shell of medium size, transversely oval in outline, usually broader 

 than long. The dimensions of two separate valves are: Length 21 mm., width 25 mm., 

 and length 14 mm., width 16 mm., the maximum width midway between the beak and 

 anterior margin, or slightly anterior to the middle. 



