MOLLUSCA. SESSILIA. 
380 
PrODUCTUSv 
20. P. plicatilis. — Transversely oblong, convex above, depressed in the 
middle, longitudinally striated and transversely wrinkled, spinose ; front 
smooth.— Min. Conch, t. cccclix. f. 2 .~ Carboniferous Limestone, 
21. P. depressa — ^Nearly semicircular, depressed, corrugated, longitudinal- 
ly striated ; upper portion convex near the beak, concave near the margin ; 
front abruptly descending. — Sower, Min. Conch, t. cccclix. f. 3. — Carboniferous 
Limestone, 
The species of this genus, appear to have a strong analogy with the Bra- 
chiopoda in the singular shagreened aspect of the shell externally, when well 
preserved ; and in the process which rises vertically, into the cavity, from 
the inside of the concave valve, besides several elevations near the hinge. It 
is probable that all the Producii and Pentameri were free shells. 
