344 Systematic Paleontology 



posterior half and the ventral in the posterior third ; the number of plica- 

 tions varies from 8 to 17. Dalman's type has from 12 to 15 plications. 

 In the Coeymans, the medial plications in nearly all the individuals attain 

 the apex of both valves, are more prominent, and in full-grown s|>ecimens 

 vary in number between fifteen and twenty. There is, therefore, a pro- 

 gressive development from the Wenlock to the Coeymans, not only in larger 

 growth, but in acquiring more plications, and these appear earlier in the 

 growth of the individuals. 



The Wenlock specimens from Dudley, England, usually attain a larger 

 growth than Gotland individuals, while both retain the same general 

 youthful expression, except that in the English material the medial plica- 

 tions appear earlier in life than in the Swedish, yet not quite so early as in 

 the Coeymans specimens. The English material, however, must be sepa- 

 rated from Dalman's species because the growth lines are farther apart, 

 become progressively stronger and finally wavy, giving the greater portion 

 of the valves a reticulated sculpture. These specimens are here named 

 Gypidula (Sieherella) dudleyensis^ 



This is the most diagnostic species of the Coeymans, occurring in great 

 numbers wherever that member is exposed. 



Occurrence. — Helde.kberg Formation, Coeymans Member. Keyser, 

 West Virginia; Dawson, Corriganville, Devil's Backbone, Tonoloway, 

 Maryland. 



Collections. — Maryland Geological Survey, U. S. National jMuseum. 



Gypidula (Sieberella) coeymanensis var. prognostica n. var. 



Plate LXII, Figs. 9-11 

 Description. — Shell subovoid to longitudinally subglobose, length 

 greater than the breadth. Ventral valve convex, quite gibbous to sub- 

 ventricose; beak pointed, arched and strongly incurved, but not closely 

 incurved over that of the opposite valve. Dorsal valve convex, subcircular 

 to transversely elliptical, slightly gibbous in the umbonal region; beak 

 incurved, extending to that of the opposite valve and beneath it. Surface 

 marked by rather indistinct plications which are more conspicuous from 



^ Pentamerus galeatus Davidson, 1867, Mon. British Foss. Bracli., Silur., p. 

 145, pi. XV, figs. 13-22 (not Dalman). 



