228 PALEOZOIC PALEONTOLOGY. 
The finer surface characters of the shell are rarely preserved upow 
the specimens studied, but different individuals, or even different 
portions of the same individual, under various conditions of exfolia- 
tion, exhibit all the types of fine surface markings figured by Hall 
for his three species. ‘These markings, as seen upon a well-preserved 
shell surface, are most nearly correctly illustrated under his S. textilis. 
‘Toward the beak the radiating markings are absent, and young shells 
preserving only this portion of the shell present the aspects of species 
of Pholidostrophia. ‘The mucronate extensions of the hinge-line are 
but rarely preserved, but the direction of the fine, concentric mark- 
ings near the hinge-line indicate their presence on most individuals. 
Perhaps the most characteristic feature of the species is the strongly 
papillose interior of the valves, which is best shown only on exfoliated 
specimens. 
LEPLTAENA RHOMBOIDALIS (Wilck.). 
Plate XX., Fig. 10. 
For complete biography see Schuchert, Bull. U. S., Geol. Surv., No- 
87, p. 240. 
Description.—Shell concavo-convex, flattened posteriorly, genicu- 
late towards the lateral and front margins, hinge-line straight, usually 
equaling the greatest width of the shell. Pedicle valve convex, beak 
small and inconspicuous. Brachial valve concave, following closely 
the curvature of the opposite one and leaving but a narrow visceral 
cavity between the two valves. Surface of both valves marked by con- 
spicuous, more or less irregular concentric wrinkles upon the 
flattened portion of the shell, and by numerous fine, radiating strie, 
which continue to the shell margin. 
The dimensions of a specimen from the coralline layer of the Decker 
Ferry formation are: length, 11 mm.; width, 16 mm. 
Remarks.—All the specimens of this cosmopolitan species which 
occur in the Decker Ferry formation are smaller than usual, with 
proportionately coarser concentric wrinkles. The specimen whose 
dimensions have been given above is one of the largest that has been 
observed. 
