380 MOLLUSCA, SESSILIA. Productus. 



20. P. plicatilis — Transversely oblong, convex above, depressed in the 

 middle, longitudinally striated and transversely wrinkled, spinose ; front 

 smooth.— Sower. Min. Conch, t. cccclix. f. 2. — Carboniferous Limestone. 



21. P. deprcssa — 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 Producti and Pentameri were free shells. 





