HYDROZOA. HYDROCORALLINA 71 



the gastropores, and dactylozooids placed in the dactylo- 

 pores. 



Millepora is an important rock-building organism at 

 the present day, often contributing largely to the forma- 

 tion of coral-reefs ; it has been recorded from Cainozoic 

 deposits, but whether these examples really belong to 

 that genus appears to be somewhat doubtful. Stylaster 

 is a living form, and is stated to occur in the Miocene. 

 Milleporidium from the Upper Jurassic of Stramberg and 

 Millestroma from the Upper Cretaceous of Egypt may 

 belong to this Order. 



ORDER V. STROMATOPOROIDEA 



In the Stromatoporoids the skeleton is calcareous 1 , and 

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

 dendroid, encrusting, or altogether irregular, and frequently 

 forms large masses. It consists of a series of concentric 

 laminae separated by spaces ; these are crossed at right 

 angles by rods or pillars, which give off horizontal pro- 

 cesses at definite intervals; these processes join together 

 and really form the laminae. The under surface of the 

 skeleton is often covered by a thin imperforate layer, with 

 concentric furrows, similar to the epitheca of many com- 

 pound corals. On the upper surfaces of the laminae there 

 are, in many forms, shallow grooves, having a stellate 

 arrangement, and known as astrorhizce. In some genera, 

 as for example Actinostroma (fig. 21), the two elements 

 of the skeleton, the laminae and pillars, remain quite 

 distinct, but in others, like Stromatopora, they become 

 to a great extent blended together so as to form a more or 



1 In some specimens the carbonate of lime has been dissolved and its 

 place taken by silica. 



