FOSSIL HYDROZOA. 



169 



This genus is likewise restricted to the earlier portion of the 

 Lower Silurian period. 



In the genus Eastrites (fig. 62), the polypary consists of a 

 slender axial tube, giving off on one side a series of linear 

 tubular cellules or " hydrothecse," which are free throughout 

 their entire length. The genus differs from all the other 

 Graptolites, in the fact that the cellules do not overlap one 

 another, but are free through their whole length, whilst it 

 is not certain if a true " solid axis " is always present. In 



Fig. 62. Morphology of Rastrites. A, Rastrites peregrinus, Barr., from the Mudstones of 

 the Coniston Series enlarged ; B, Rastrites capillaris, Cam, from the Upper Llandeilo Shales 

 of Dumfriesshire enlarged ; c, Fragment of Rastrites Linncti, Barr., from the Coniston Mud- 

 stones enlarged ; D, Fragment of R. peregrinus, greatly enlarged, showing the impressed 

 line running up the centre of each cellule. (Original.) 



Britain and North America the species of fiastrites are ex- 

 clusively confined to the Lower Silurian rocks, but in 

 Bohemia they pass up into the lowest beds of the Upper 

 Silurian. 



In the genus Diplograptus (fig. 63), the polypary consists 

 of two simple monoprionidian stipes, firmly united to one 

 another, back to back. The frond, therefore, is "diprioni- 

 dian," or carries cellules on both sides. The solid axis is 

 usually prolonged beyond the base of the polypary as a 

 longer or shorter process or " radicle," which is often flanked 

 by lateral spines. The solid axis is also almost invariably 

 prolonged beyond the opposite or " distal " end of the poly- 

 pary as a naked rod. In the nearly-allied genus Climaco- 



