170 



CCELENTERATA. 



graptus, the structure is much as above described, but the 

 cellules have such a structure that their mouths appear to 

 be sunk below the general surface of the polypary, forming 

 a row of rounded or quadrangular openings on each side. 



Both Diplograptus and Clwiaco- 



m graptus range in Britain and North 

 I rL America from the base to the sum- 

 mit of the Lower Silurian series ; 

 but in Bohemia they rise into the 

 lower portion of the Upper Silu- 

 rian deposits. In the genus Dic- 

 ranograptus the polypary is at first 

 diprionidian, but soon splits into 

 two monoprionidian branches which 

 carry the cellules along their outer 

 margins. The genus is exclusively 

 Lower Silurian. Lastly, we may 

 mention here the curious forms in- 

 cluded under the generic title of 

 Phyllograptus (fig. 64). In these 

 forms, which are essentially charac- 

 teristic of the lowest Silurian de- 

 posits (Skiddaw and Quebec groups), 

 the polypary is leaf-like in shape, 

 and consists of four rows of cellules 

 placed back to back, thus resem- 

 bling two Diplograpti intersecting 

 each other at right angles. In 

 consequence of the peculiar struc- 

 ture of the polypary, the Phyllo- 

 grapti are sometimes spoken of as the " tetraprionidian " 

 Graptolites. 



IV. SUB-CLASS HYDROCORALLIK&. This name has re- 

 cently been proposed by Mr Moseley for two groups of 

 marine animals which produce a regular skeleton of car- 

 bonate of lime, often of large size, and which have been gen- 

 erally referred to the Corals (Actinozoa). One of these groups 

 comprises the well-known Millepora, (fig. 65), which is found 

 contributing so largely to the formation of coral-reefs in the 



Fig. 63. A, Diplograptus pristis, 

 His. , slightly enlarged, showing the 

 normal condition of the base ; B, 

 Another example of the same, 

 slightly enlarged, showing a long 

 radicle, and long lateral spines ; c, 

 Another of the same, enlarged, 

 showing lateral spines, succeeded 

 proximally by a small bulb, but 

 showing no true radicle. (Ori- 

 ginal.) 



