150 RAMBLES AND REVERIES. 



or skeletons of the polypes, with the common 

 calcareous basis uniting the corallites together and 

 called " coenenchyma," thus corresponding to the 

 ccenosarc which joins the soft polypes to one 

 another. 



Occasionally a coral deposit will be found in a 

 very friable state, and the fossils will occur as casts 

 or impressions only. The lime which originally be- 

 longed to the fossils has been gradually dissolved 

 out by rain-water penetra- 

 ting into the rocks. Many 

 such casts of Favosites, 

 Cyathophyllum, etc., may 

 be got near Windermere. 

 The foregoing remarks 

 will make it abundantly 

 clear that the Silurian 

 period was one of profuse 

 coral development ; indeed 

 FIG. 38.-Magnified corallites no fewer than seventy-six 



of H. interstinctus. f i . V 



species of corals have been 

 described from the Wenlock formation alone. 



This applies equally to the Devonian rocks of this 

 country. From these strata fifty-two species of corals 

 have been obtained. The Plymouth limestones are 

 regarded by geologists as portions of an ancient 

 coral-reef which fringed the earlier Cambrian and 

 Silurian rocks. Reef-building corals are abundant 

 in them. It is not always easy to determine which 

 are reef-builders, but there is no doubt as to the 

 Astrceae (Fig. 36) being amongst them, for they are 

 still at work in many parts of our globe, and these, 



