204 THE DEVONIAN OF MISSOURI. 



very abruptly to the baso-lateral margins; beak closely incruved beneath that of the 

 opposite valve. Surface marked by obscure fine radiating striae, Which are crossed by 

 indistinct concentric lines of growth; the latter often becoming conspicuous towards 

 the margins. 



''[Remarks] — This * * * * species differs considerably in the form and 

 depth of the mesial sinus * * . In some of the specimens the sinus is very 

 shallow and flattened within, while in other instances it is more impressed and angular 

 in the middle; other specimens present intermediate grades of difference in this respect, 

 the sinus being nearly flat within, and marked by a narrow, nearly linear, deeper de- 

 pression along its middle * * * * * * * 



********* Hall 1XSQ 



Remarks — This species is represented by a single, incomplete shell. 



Meristella atoka Girty 

 Plate 53, figures 7-20 



Meristella arcuata var. atoka Girty, 1899, U. S. Geol. Surv., 19th Ann. Rept., pt. Ill 



p. 567, pi. LXXI, figs. la-f. 

 Meristella atoka Dunbar, 1919, Tenn. Geol. Surv., Bull. 21, pi. 1, fig. 11; pi. 2,, fig. 3. 



Description — "Shell rather small, subtriangular; width slightly greater than the 

 length. 



"Umbo of the ventral valve full; beak small and strongly incruved; sinus strongly 

 marked, beginning above the middle of the shell as a depressed line, deepening and 

 broadening as growth advances, but never losing its angular character, the upturning 

 of the sinus towards the front and the angularity so pronounced as often to make a 

 distinct emargination in the anterior outline, which would otherwise be well rounded. 

 Young examples are subcircular, but usually with a faint angular sinuation, noticeable 

 in the two valves as seen from the front. 



"From this type, by gradations, considerable variations are found. The fold varies 

 both in strength and shape, and specimens occur in which this feature is high and 

 rounded as well as high and angular, and others in which it is low and rounded and low 

 and angular. But generally it is both angular and well marked. Similarly the shape 

 varies from being wider than long to being longer than wide, the beak of the ventral 

 valve in that case being higher and more projecting. Perhaps several specific names 

 might be employed for the extremes of variation, but I do not believe that these distinc- 

 tions would be justified * * * * * 



* * * * * * * —Girty, 1899 



Remarks — A description of the Missouri representatives follows: 



Shell subtriangular to subpentagonal in outline, anterior border emarginate. . 

 Ventral valve longitudinally arcuate, medial anterior portion prolonged dorsally; 

 marked, medially, from anterior margin posteriorly with prominent, subangular sinus; 

 beak incurved over that of dorsal valve. Dorsal valve marked, medially, from anterior 

 margin posteriorly with prominent, rounded fold; beak incurved. Surface ornamented 

 with radiate costae — 4 in 1 mm. near anterior border of shell — crossed by concentric 

 growth lines. 



