394 Dr. H. A. Nicholson and Mr. A. H. Foord on a 



Ohs. As regards the mode of growth in this form, the 

 corallum not unfrequentlj consists of a single layer of coral- 

 lites, perhaps ^ to 1 centim. in thickness, and forming an 

 expansion of some inches across. In such a case the coral- 

 lum may have a basal epitheca or it may be cemented down 

 to some foreign body*. In a great many examples the 

 corallum consists of a number of successively superposed 

 layers or colonies, which may be all alike or which may differ 

 from one another in certain apparently structural features. 

 Sometimes these successive layers are obviously only the result 

 of progressive interruptions in the growth of a single coral- 

 lum, just as is seen in many other corals. In other cases the 

 successive layers are separated by a complete interruption of 

 continuity, a minute interval, represented by a delicate layer 

 of mineral matter, existing between each pair of contiguous 

 corallites. In such cases, provided the successive layers are 

 all alike in structure, we see no reason to doubt that they 

 belong to a single species and are the result of the growth of 

 a number of successive generations one above the other. Pre- 

 cisely the same phenomenon is to be seen in many Stromato- 

 poroids, in which the coenosteum consists of a number of 

 " latilaminaj," which may be separated from one another in 

 places by more or less complete breaks or intervals. 



In other specimens the fossil not only consists of a number 

 of superimposed layers or colonies, but one or more of these 

 layers may present appearances very different from the rest, 

 the nature of which will be subsequently explained. In such 

 cases it is natural to suppose that the differing layers belong 

 to different species. Thus it is very common to meet with 

 specimens composed of one or more layers of corallites which 

 are distinguished by having their visceral chambers occupied 

 by clear calcite and their walls and tabulte distinct, together 

 with one or more layers in which the visceral chambers are 

 more or less completely obliterated by the removal of the 

 tabulae and the walls undiscernible. 



Professor Schliiter has regarded such cases as the result 

 of the parasitism of one species upon another distinct species. 

 Hence he has called the layers with normal corallites and 

 visceral chambers Calamopora piliformis^ and he has given 

 the title of PachytJieca stellimicans to the layers in which 

 the corallites have their walls and visceral cliarabers more or 

 less extensively effaced in a manner to be subsequently 

 described and explained. 



The observations we have made upon a very extensive 



* It occurs very commonly growing' upon the laminar coenosteum of 

 Stromatoporella eifeliensis, Nicli. 



