236 PALEOZOIC PALEONTOLOGY. 
Remarks—In some of its characters this species resembles the 
Helderbergian forms Uncinulus mutabilis H. and U. nucleolata H. 
It differs from both of them, however, in its smaller size, and in the 
obsolescence of its plications on the posterior portions of the shell. 
Internally there are conspicuous differences which are of even generic 
value. In Wilsonia globosa the cardinal process is absent, the hinge- 
plate is divided and is supported by a strong median septum. These 
-eharacters, when associated with its external form, place it in the 
genus Wilsonia, while the two Helderbergian species mentioned possess 
well-developed cardinal processes and the other characters which dis- 
tinguish the genus Uncinulus. 
ATRYPA RETICULARIS (Linn.). 
Plate XXI., Figs. 35-37. 
189%. Atrypa recticularis Schuchert, Bull. U. 8S. Geol. Surv., No. 87, 
p. 154. A complete bibliography of this species may be 
found in this place. 
Deseription.—Shell subcircular or subelliptical in outline and sub- 
hemispherical in general form, hinge-line shorter than the greatest 
breadth. Pedicle valve nearly flat, slightly convex on the umbo but 
concave toward the margins, beak small. Brachial valve strongly 
convex or gibbous, the greatest elevation near the centre, from which 
point it curves down regularly to the margins on all sides, the beak’ 
strongly ineurved under the beak of the opposite valve. Surface of 
both valves marked by coarse, rounded plications about 1 mm. apart 
at the margin of the shell, which increase by intercalation and by 
division; also marked by strong, concentric, lamellose extensions of the 
shell, which are usually obliterated by exfoliation, their position being 
indicated by concentric ridges. 
‘Fhe dimensions of an average specimen are: length, 22 mm.; 
width, 22 mm.; convexity of brachial valve, 10 mm. 
Remarks.—This cosmopolitan species is often exceedingly abundant 
in. the lower beds of the Decker Ferry formation, some strata being 
made up almost exclusively of the crushed and broken shells. The 
particular variety of the species present is a rather coarsely-marked 
form, with exceptionally flat pedicle valve and correspondingly gibbous 
