8 4 



CCELENTERATA. 



in a species of corneous " disc" or plate, which is believed 

 to have been composed of two laminae. The functions of 

 this disc are doubtful ; but it has been compared with the 

 " float " or buoy of the Physophorida, an order of the Oceanic 

 Hydrozoa. 



In the genus Rastrites (fig. 34), the polypary consists of a 



Fig. 34. Morphology of Rastrites. A, Rastrites peregrinus, Barr., from the Mud- 

 stones of the Coniston Series, enlarged. B, Rastrites capillaris. Can-., from the Upper 

 Llandeilo Shales of Dumfriesshire, enlarged. C, Fragment of Rastrites Linruri, Barr., 

 from the Coniston Mudstones, enlarged. D, Fragment of R. peregrimis, greatly en- 

 larged, showing the impressed line running up the centre of each cellule. (Original.) 



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

 lar 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 very doubtful if a 

 true " solid axis " is present. In Britain and North America 

 the species of Rastrites are exclusively confined to the Lower 

 Silurian Rocks, but in Bohemia they pass up into the lowest 

 beds of the Upper Silurian. 



In the genus Diplograpsus (fig. 35), the polypary consists of 

 two simple monoprionidian stipes, firmly united to one another, 

 back to back. The frond, therefore, is "diprionidian," 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 op- 

 posite or " distal " end of the polypary as a naked rod. In the 

 nearly-allied genus Climacograpsus, the structure is much as 

 above described, but the cellules have such a structure that 



