200 
GEOLOGY OF THE FOURTH DISTRICT. 
The following illustration presents some of the typical fossils of the Moscow shale. Seve¬ 
ral of these have not been seen in the group below while others do occur in lower situations. 
80. 
1, 1 a, 1 b, 2. Atrypa spinosa. 5. Delthyris zigzag. 
3. Atrypa concinna. 6. Calymene bufo. 
4. Strophomena inequistriata. 
7. Cryphaeus calliteles. 
8. Loxonema sinuosa. 
1 & 2. Atrypa spinosa, n. s.—Sub-orbicular, often flattened, producing an angle at the extre¬ 
mities of the hinge ; beak of the lower valve slightly prominent, incurved ; surface with about 
twenty rounded radii, which bifurcate often very regularly at less than half the distance from 
beak to base; radii crossed by numerous elevated lamellae, which upon each rib are folded 
and extended into a spine of one-fourth to one half an inch in length; surface between the 
lamellae concentrically striated. 
The shell more commonly appears without the spines, or with them partially worn off. 
When destitute of spines, it resembles the figure of A. squamosa (Sowerby, Geol. Trans. 
2d series, Vol. 5, pi. 57, fig. 1). 
Fig. 1 is of a specimen from Eighteen-mile creek, where the spines are always absent. 
Fig. 2 is of a specimen from Moscow, where the spines are more or less perfect, being en¬ 
cased around the margin in soft shale, and worn off on the prominent part of the shell. 
This fossil is frequently confounded with Atrypa prisca. 
Localities —Moscow, Eighteen-mile creek. 
3. Atrypa concinna, n. s. — Lenticular, nearly smooth, or with a few distant elevated lines 
of growth; lower valve most convex, with a faintly impressed line extending from beak to 
base ; beak of the upper valve scarcely prominent, and closely pressed within the lower one ; 
beak of lower valve prominent, small, acute, incurved. 
