28 The Fauna of the Keyser Member of the 



Superfamily SPIRIFERACEA 

 Family ATRYPIDAE _ 



Genus ATRYPA Dalman 

 Atrypa recticularis (Linne) \ 



Plate LXVIl, Figs. 26-28 



Anomia reticularis Linne, 17C7, Systema Naturae, ed. xii. tome i, p. 1132. 

 Atrypa reticularis Hall, 1852, Nat. Hist. N. Y., Pal., vol. ii, p. 72, pi. xxiii, fig. 

 8; p. 270, pi. iv, fig. 5. 



Description. — " Shell subrotnnd, more or less compressed, subtruncated 

 above or on the hinge-line ; valves more or less equal, the beak of the dorsal 

 valve extending beyond the ventral valve, and the latter being deeper and 

 more convex in older specimens ; surface marked by dichotomous rounded 

 strias, which are crossed by concentric elevated lamellae, giving a reticu- 

 lated 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 with various modifications 

 as far as the Chemung. In each of its geological positions, however, it 

 presents peculiar characters and we are able to decide at once the geological 

 position of specimens by these peculiarities. On its first appearance in 

 the Clinton group, it shows its variable character in a remarkable degree, 

 and it is scarcely possible to avoid referring the individuals to distinct 

 species. In many of the young specimens the ventral valve is nearly flat, 

 or slightly convex, with a depression along the center from beak to base. 

 In specimens of medium size the valves are nearly equal and in older ones 

 the ventral valve is the more convex. Again there are others where, in the 

 young shell the ventral valve has no depression in the center, and is equally 

 convex with the dorsal valve. In the radiating strise or plications it is 

 equally variable; many specimens having them very distinctly dichot- 

 omous, while others are nearly undivided from the beak. In many young 



^ For figures of the Upper Silurian and Lower Devonian variation see Hall, 

 1859, Nat. Hist. N. Y., Pal., vol. iii, pi. xlii, figs. la-r. For a complete American 

 bibliography and synonymy see Echuchert, 1897, Bull. U. S. Geol. Survey, No. 

 87, pp. 154-155. 



