70 HYDROZOA. GRAPTOLITOIDEA 



part, and the genera Nemagraptus, Dicellograptus, Dicrano- 

 graptus, Diplograptus and Climacograptus now appear for 

 the first time. In the Bala Beds, the four last-mentioned 

 genera become much more abundant, and with them occur 

 Leptograptus and Pleurograptus. The only genera which 

 pass up from the Bala to the Silurian are Climacograptus, 

 Diplograptus, and Retiolites ; not a single Bala species 

 (except perhaps a variety of Climacograptus scalaris) is 

 found in the Llandovery Beds, so that between the 

 Ordovician and Silurian there is a great break in the 

 graptolitic succession. As a whole, the Silurian formations 

 are characterised by the presence of the genera Mono- 

 graptus, Rastrites and Cyrtograptus, which appear first 

 at the base of the Llandovery Beds. In the lower part 

 of the Llandovery the genera Diplograptus and Climaco- 

 graptus are fairly abundant, but they become extinct in 

 the Tarannon, and in the Wenlock and Ludlow Beds the 

 only forms are Monograptus, Cyrtograptus, and Retiolites. 

 The last traces of graptolites occur in the Downtonian 

 Beds, but they are too imperfect for determination. 



ORDER IV. HYDROCORALLINA 



The skeleton in the Hydrocorallina is calcareous and 

 has the form of encrusting or branching masses. It con- 

 sists of a network of rods, in which there are tubes of 

 two sizes opening on the surface ; the larger are called 

 gastropores, and have horizontal partitions or tabulae ; the 

 smaller are named dactylopores. The skeleton is of ecto- 

 dermal origin, and is secreted by a network of coenosarcal 

 tubes, above which is a superficial layer of ectoderm. The 

 polvps project above this layer, and are of two kinds : 

 nutritive individuals or gastrozooids, which are placed in 



