X FOSSIL CORALS STROMATOPORIDAE 283 



character of these tubes, their very minute si;^e, the absence of 

 ampullae, and the general texture of the corallum, are characters 

 which separate this fossil ybyj distinctly from any recent 

 Hydroid corals. Porosphaera, therefore, was probably not a 

 Hydrozoon, and certainly not related to the recent Millepora. 



Closely related to Porosjjhacra apparently are other globular, 

 ellipsoidal, or fusiform corals from various strata, such as LofUisia 

 from the Eocene of Persia, Parheria from the Cambridge Green- 

 sand, and Heterastridium from the Alpine Trias. In the last 

 named there is apparently a dimorphism of the radial tubes. 



Allied to these genera, again, but occurring in the form of 

 thick, concentric, calcareous lamellae, are the genera Ullij^sactinia 

 and Sphaeractinia from the Upper Jurassic. 



Another important series of fossil corals is that of the family 

 Stromatoporidae. These fossils are found in great Iteds of 

 immense extent in many of the Palaeozoic rocks, and must have 

 played an important part in the geological processes of that 

 period. They consist of a series of calcareous lamellae, separated 

 by considerable intervals, encrusting foreign bodies of various 

 kinds. Sometimes they are fiat and plate -like, sometimes 

 glolnilar or nodular in form. The lamellae are in some cases 

 perforated by tabulate, vertical, or radial pores, but in many 

 others these pores are absent. The zoological position of the 

 Stromatoporidae is very uncertain, but there is not at present 

 any very conclusive evidence that they are Hydrozoa. 



Stromatopora is common in Devonian and also occurs in 

 Silurian strata. Cannop)ora from the Devonian has well-marked 

 tabulate pores, and is often found associated commensally with 

 another coral {Aulopora or Syringoporci). 



Order VI. Stylasterina. 



The genera included in this order resumWe Millepora in pro- 

 ducing a massive calcareous skeleton, and in showing a consistent 

 dimorphism of the zooids, but in many respects they exhibit 

 great divergence from the characters of the Milleporina. 



The colony is arborescent in growth, the branches arising 

 frequently only in one plane, forming a flabellum. The cal- 

 careous skeleton is perforated to a considerable depth by the 

 gastrozooids, dactvlozooids, and nutritive canals, and the gastro- 



