LUCERNARIDA AND GRAPTOLITIDiB. 83 



Most of them, also, exhibit a very anomalous and remarkable structure, 

 termed the " solid axis." This is a peculiar fibrous rod, which no doubt 

 served to strengthen the polypary, and which is often prolonged beyond one 

 or both ends of the polypary in a naked state. There is also good evidence 

 that the reproductive process in the Graptolites was carried on in a manner 

 somewhat similar to what is seen in the living Sertularians — ^namely, by 

 means of reproductive buds enclosed in horny capsules. Graptolites most 

 usually present themselves as beautiful silvery impressions, covering the sur- 

 face of the black shales of various parts of the Silurian system. 



