218 



HYDROZOA. 



freely. The species of both these genera are confined to the 

 Ordovician rocks. 



Coming next to the group of the Diprionidian Graptolites, the 

 typical family is that of the Diplograptidce, of which the type-genus 

 is Diplograptus itself. In this genus the polypary (fig. 104) con- 

 sists — as it does in most of the genera of the family — of two sim- 

 ple monoprionidian branches, 

 which are closely united to 

 one another along their flatten- 

 ed dorsal surfaces, but are en- 

 closed in independent perider- 

 mal sheaths, and are therefore 

 really separate. The virgulae 

 of the two branches coalesce, 

 and form a single rod-like 

 axis, which is embedded in a 

 groove between the ccenosar- 

 cal sheaths, and commonly 

 projects distally as a naked 

 fibre, while it appears proxi- 

 mally as a shorter or longer 

 "radicle" (figs. 104 and 105). 

 In some of the Difilograptidce, 

 however, according to Lap- 

 worth, there is only a single 

 ccenosarcal canal from which 

 the two rows of hydrothecse 



Fig. 102. — Dichograptus octobrachiatus, showing the central disc. (After Hall.) 

 Ordovician (Skiddaw and Quebec groups). 



spring. The hydrothecse themselves overlap each other to a greater 

 or less extent, the distal portion of each being free. The species of 

 Diplograptus are found in both the Ordovician and Silurian rocks. 

 In the nearly allied genus Climacograptus (fig. 105), the structure is 

 much the same as in Diplograptus, but the hydrothecse are vertically 

 disposed, in such a manner that their mouths appear to be sunk 

 below the general surface of the polypary. The appearances pre- 

 sent by the polypary in this genus vary extremely, according to 



