46 HYDROZOA. GRAPTOLITOIDEA. 



followed by other buds, which in like manner pass into 

 hydrothecse. In the diprionidian forms the buds are 

 produced on two sides of the sicula ; they are sometimes 

 directed towards the point of the sicula, at other times 

 away from it. When two series are formed they may 

 either remain free or be united by their dorsal surfaces. 

 Owing to the fact that the soft parts of the graptolites 

 are entirely unknown, it is difficult to speak of their 

 affinities with any degree of certainty. There can be no 

 doubt, however, that they belong to the Hydrozoa ; and 

 Allman and others have referred them doubtfully to the 

 Hydromedusa3, being nearest to such forms as Sertularia 

 and Plumularia, with which they agree in the general 

 arrangement of the hydrothecae, and in having gonangia. 

 But they differ from them in possessing a virgula, in 

 being free forms, in the diminution in size of the hydro- 

 thecse towards the proximal end of the polypary, and in 

 the free communication which exists in many, if not all, 

 cases between the hydrothecae and the common canal. 

 The graptolites also resemble slightly some forms of 

 Polyzoa, as for instance Rhabdopleura, which possesses a 

 chitinous axis. 



Monograptus. (fig. 8 e.) Polypary simple, unilateral, 

 monoprionidian, straight, curved or spiral. Hydrothecse 

 vary in form in different species. The pointed end of 

 the sicula is thrown back upon and attached to the dorsal 

 side of the proximal end of the polypary. Lower Llan- 

 dovery to Lower Ludlow. 



Rastrites. Polypary similar to Monograptus, but 

 the common canal is very narrow, and the hydrothecffi are 

 long and widely separated. Lower Llandovery to Tarannon. 



Cyrtograptus. Polypary similar to Monograptus, but 



