818 Report oF THE StatE GEOLOGIST. 
oped adult conditions this thickening is absent, the extremities 
of the processes are in close apposition, or may form a continu- 
ous lamella. The muscular impressions consist of four large 
adductor scars divided by a low median ridge. 
Ovarian pittings and vascular sinuses occur over the inner 
surfaces of both valves. The latter consist of two main trunks, 
sending two branches posteriorly, and two longer converging 
branches anteriorly. 
External surface covered with radial plications crossed by 
concentric growth-lines; at the crossing of the two series of 
lines the external layers of the shell may be produced into 
broad lamellar expansions or hollow spines. 
. Shell-substance fibrous, impunctate. 
Type, Atrypa reticularis, Linné (sp.). 
Distribution. Upper Silurian — Lower Carboniferous. 
Subgenus Gruenewaldtia, Tschernyschew. 1885. 
. (Plate 41, figs, 21, 22.) 
Atrypoid shells having the relative convexity of the valves in 
A. reticularis reversed. Spiral cones with their bases lying 
against the lateral slopes of the pedicle-valve, the outer face of 
the cones being thus parallel to, and just within the surface of 
Fia. 400. Fie. 401. 
Spirals of Gruenewaldtia latilinguis, Schnur, In fig. 401, the pedicle-valve is the lower and 
the two median dots represent sections of the prinary lamellz, (TscH@RNYSCHEW .) 
the brachial valve. It is such a modification of the brachial 
apparatus as must necessarily ensue from the variation in the 
contour of the shell. The character of the jugum has not been 
determined. 
(Type, Gruenevalatia latilinguis, Schnur (sp.). Lower and 
Middle Devonian.) 
, = a = 
