PENTAMERUS POLYPARIA. 



its place longer than any other. From the incalculable antiquity 

 of their lineage, the terebratulae have been humorously styled the 

 Fossil aristocracy. 



34. The genus Pentame'rus (Jigs. 15, 16, 17 from the Greek 

 pente, five, and meros, parts, or cells), contains four known species 

 all of which belong to the Silurian rocks. In 

 this genus, the lesser valve is divided inter- 

 nally by two parallel walls, or septa, running 

 close together lengthwise along the shell, 

 forming three cells ; the other valve also has 

 a septum or wall, which is forked towards the 

 beak of the shell, and divides it into two cells ; 

 thus forming the five cells to which it is in- 

 debted for its generic name. The casts of 

 these shells (fig* 15), often have fissures, pro- 

 duced by the decomposition of the septa ; and occasionally these 

 cavities are occupied by calcareous spar. 



Fig. 1.5. Cast of the 

 Pentamerus Icevis. 



Fig. 16. Pentame'rus Knightii. Fig. 17. Section of same Shell. 



35. Of the polypa'ria or corals which existed when the SiliTJar 



Fig. 18-Cyatkophyllum turbinatum. Fig. 19.Catenipo~a escharoides 



rocks were formed, representations of two genera are given. The 



34. How is the genus pentame'rus characterized ? 



35. Did corals exist in the Cambrian and Silurian rocks ? 



24 



