LUCERNARIDA AND GRAPTOLITID. 89 
known Sertularians in the fact that the hydrosoma was not fixed to any 
solid object, but was permanently free. Most of them, also, exhibit 
a very anomalous and remarkable structure, termed the ‘‘solid axis ” (fig. 
31, 3). This isa 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 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 surface of the black shales of various parts of the Silurian system. 
