FOSSIL HYDROZOA. 



77 



thecce" or little cup-like expansions derived from the polypary. 

 The process of reproduction is carried on by the development of 

 the reproductive elements within horny urn-like sacs, which are of 

 larger size than the " hydrotheccz" and are known as " ovarian 

 capsules" or " gonotheccz" (often called " gonophores"}. Type oj 

 the order, the Sea-fir (Sertrtlaria, fig. 24). 



As in the case of the Corynida, there is some uncertainty as 

 to the existence of any fossil representatives of this order. No 

 undoubted Sertularian, at any rate, is as yet known to the 

 palaeontologist ; but there are several genera which may with 

 more or less probability be referred to this place. The most 

 important of these as being those in which the reference is 

 probably correct are certain forms usually referred to the 



Fig 25 Dendrograpsns Hallianus. a Portion of the frond, natural size; b Por- 

 tion of a branch, enlarged ; c The footstalk and some of the principal branches, natural 

 size. (After Hall.) 



Graptolitida, of which the genera Dendrograpsus and Dicty- 

 oncma may be noticed in particular. The forms referred to 

 Dendrograpsus are exclusively confined to the Upper Cam- 

 brian and Lower Silurian formations. They consist of plant- 

 like spreading and branched growths, which are furnished with 

 a strong footstalk (fig. 25). In all probability the organism 

 was attached by the base of the footstalk to some foreign 

 body, but no actual demonstration of this has as yet been 

 obtained. The branchlets carry upon one side a series of 



