HYDROZOA. STROM ATOPOROIDEA. 49 



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 an enormous break in the graptolitic 

 succession. As a whole the Silurian formations are 

 characterised by the presence of the family Monograp- 

 tidce 1 , which appears first at the base of the Llandovery 

 Beds. In the lower part of the Llandovery the genera 

 Diplograptus and Climacograptus are fairly abundant, but 

 they become extinct in the Tarannon, and in the Wenlock 

 and Ludlow Beds the only forms are Monograptus, Cyrto- 

 graptus, and Retiolites. The last traces of graptolites 

 occur in the Downtonian Beds, but they are too imperfect 

 for determination. 



SUB-CLASS. STROMATOPOROIDEA. 



In the Stromatoporoids the skeleton is calcareous, and 

 very variable in form ; it may be hemispherical, spheroidal, 

 dendroid, encrusting, or altogether irregular, frequently 

 forming large masses. It consists of a series of concentric 

 laminae separated by interlaminar spaces ; both are crossed 

 at right angles by a series of rods or pillars, which give off 

 horizontal processes at definite intervals, and these joining 

 together really form the porous lamina. The stromatopo- 

 roids were compound animals, the polypites in some cases 

 simply occupying pores in the external lamina, in others 

 there were definite tubes. The Stromatoporoids are 

 most closely related to such living forms as Hydractinia 



1 This includes the genera, Monograptus, Rastrites and Cyrtograptus. 



w. p. 4 



