198 THE DEVONIAN OF MISSOURI. 



by the deltidium. Dorsal valve flattened or depressed convex; beak scarcely elevated 

 above the hinge; hinge line sloping from the beaks at an angle of about 150 degrees, 

 rounded at the extremities, nearly equal to the greatest width of the shell; false area 

 narrow, much shorter than the hinge. Surface marked by ten to twelve plications 

 on each valve, of which two on the middle of the ventral valve are larger and more 

 elevated than the others, and separated by a wider and deeper depression than between 

 those on each side. The central plication on the dorsal valve is larger than the others — 

 near the front, but usually dies out before reaching the beak. Shell marked by strong 

 imbricating concentric lamellae of growth." — Hall, 1859. 



Remarks — The observed specimen has 6 plications on each valve. The depression 

 between the two central plications of the ventral valve is narrower than the depressions 

 that bound them. The central plication of the dorsal valve is depressed, is narrower than 

 the plication on each side of it, and does not attain the beak of the valve. 



Genus Atrypa Dalman 



Atrypa reticularis (Linne) 



Plate 50, figures 30-40 



Anomia reticularis Linne, 1767, Systema Naturea, ed. XII, I, p. 1132. 



Atrypa reticularis Hall, 1852, Pal. N. Y., vol. II, p. 72, pi. XXIII, fig. 8; p. 270, pi. LV. 



fig. 5. 

 Atrypa reticularis Hall, 1859, Pal. N. Y., vol. Ill, p. 253, pi. XLII, figs. 1 a-r, 1861. 

 Atrypa reticularis Keyes, 1894, Mo. Geol. Surv., vol. V, p. 97, pi. XLI, figs. 13a, b. 1 

 Atrypa reticularis Girty, 1899, U. S. Geol. Surv., 19th Ann. Rept., pt. Ill, p. 565. 

 Atrypa reticularis Weller, 1903, Pal. N. J., vol. Ill, pp. 236, 286, pi. XXI, figs. 35-37; 



pi. XXX, figs. 11-18. 

 Atrypa reticularis Shimer, 1905, N. Y. State Mus., Bull. 80, p. 246. 

 Atrypa reticularis Clarke, 1908, N. Y. State Mus., Mem. 9, pt. 1, p. 109. 

 Atrypa reticularis Clarke, 1909, N. Y. State Mus., Mem. 9, pt. 2, pp. 42, 81. 

 Atrypa reticularis Maynard, 1913, Md. Geol. Surv., L. Dev., p. 392, pi. LXVII, figs. 



26-28. 



Description — "Shell subrotund, more or less compressed, subtruncated above or 

 on the hinge line^ valves more or less equal, the beak of the dorsal [ventral] valve ex- 

 tending beyond the ventral [dorsal] valve, and the latter being deeper and more convex 

 in older specimens; surface marked by dichotomous rounded striae, which are crossed 

 by concentric elevated lamellae, giving a reticulated or decussated character to the 

 surface. 



"It is impossible to give a definite description of this very protean species, which 

 commences its existence in the Clinton group, and continues under various modifica- 

 tions through the succeeding strata as far as the Chemung group. In each of its geolo- 

 gical positions, however, it presents peculiar characters, and we are able to decide at 

 once the geological position of specimens, by these peculiarities. * * 



**** *** * _Hall, 1852. 



Remarks — The variations exhibited by the representatives at hand from the 

 Missouri Helderberg are shown by the specimens figured. 



»See Schuchert, C , U. S. Geol. Surv., Bull. 87, pp. 154-155, 1897, for complete 

 bibliography to 1897. 



