LUCERNARIDA AND GRAPTOLITID.E. 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. 



