214 



HYDROZOA. 



very usual, as pointed out by the writer, to find in association with 

 Graptolites peculiar, bell-shaped or conical, chitinous capsules, which 

 may be spoken of by the general name of Dawsonice (fig. 95). These 

 capsules present different appearances in accordance with the direc- 

 tion in which they have been compressed, and they vary in shape 

 and size. Each is furnished with a little spine or mucro at its sum- 

 mit, and also with a marginal chitinous thread or fibre. These 

 singular bodies have not hitherto been certainly detected in direct 

 connection with the polypary of any Graptolite. They may, however, 



Fig. 95. — Various forms of bell-shaped chitinous capsules (Dazvsonici) found associated with 

 Graptolites, and supposed to be of the nature of" gonangia. " (Original.) 



be regarded, with great probability, as being of the nature of " gon- 

 angia," though they probably differed from the structures so called 

 in the recent Sertularians in becoming detached from the parent 

 colony. 



Two leading types of structure may be distinguished amongst the 

 Graptolites, and these may be respectively termed the " monoprioni- 

 dian " and " diprionidian " type. The monoprionidian Graptolites 

 (MonoprionidcR) are distinguished by the fact that, whether the 

 colony be simple or branched, the hydrothecas are arranged in a 

 single series, and the ccenosarc is thus single and not double. In 

 the typical Monoprionidce the hydrothecse are uniserial over the 

 entire polypary (as in Monograptus or Didymograptus). A transition 

 is, however, effected between the monoprionidian and diprionidian 

 groups of Graptolites by such intermediate types as Dimorpho- 

 graptus (fig. 92, f) in which the proximal portion is monoprioni- 

 dian and the distal portion diprionidian, or Dicranograptus (fig. 

 103, a) in which the opposite state of things obtains. On the other 

 hand, in the proper Diprionidce the polypary is duplicate, and con- 

 sists of two separate rows of hydrothecae, which may spring from an 

 undivided common ccenosarc, but which, more usually, arise from 



