510 



BESCEIPTION OF 



\PolygaKlrica. 



It exactly resembles a perfectly annular Spongiolithis hrachiata, 

 and is a doubtful member of the genus Mesocena. The Spongiolithis 

 belongs to the class Phjtolitliaria. 



Mesocena hinonaria. — A slender ring with nine angles, each armed 

 with a denticle, and these alternating with the same number of den- 

 ticles on the inside. Diameter l-768th. Fossil in Peruvian guano. 



M. hiodonaria. — A slender ring with eight exterior spines and 

 angles, and as many alternating internal denticles. Diameter 1-76 8th. 

 Fossil in Peruvian guano. 



M. heptagona. — Thering with seven external teeth. Peru (P. 1 5, f. 71 .) 



M. odogona. — The ring with eight external teeth or spines. Peini. 



