72 



HYDROZOA. STROMATOPOROIDEA 



less netted structure ; between these two types, however, 

 there are intermediate forms. The first type (Actino- 

 stroma) is similar to Hydractinia (see p. 53), but is always 

 calcareous and forms larger masses ; the second (Sfrornato- 

 pora) resembles Millepora (see p. 71), and, like that genus, 

 possesses tubes with horizontal partitions, but the tubes 

 seem to be of one size only, and consequently there is 

 nothing to indicate that this type was dimorphic; it 

 differs also in possessing radial pillars. 



c D 



Fig. 21. A, Tangential section of Actinostroma intertextum showing the 

 radial pillars. B, vertical section showing the radial pillars and the 

 formation of the concentric laminse by processes given off from 

 these, x 12. C and D, parts of A and B further enlarged. From 

 the Silurian Rocks. (After Nicholson.) 



The soft parts in the Stromatoporoids probably formed 

 a continuous layer on the surface of the skeleton, and the 

 polyps in some cases (e.g. Stromatopora) were placed in 



