148 RAMBLES AND REVERIES. 



that the ancient tabulate fossils, so abundant in 

 Silurian and Devonian rocks, are composed of calcite, 

 whereas modern corals are formed of Arragonite, 

 a not very different material, though somewhat 

 differently built up. 



Let us now turn our attention to the geological 

 distribution of corals. Their earliest occurrence is 

 in the Silurian rocks, in which Favosites Gothlandica 

 (Fig. 32, 33) and Halysites 

 catenulatus (Fig. 35) are 

 frequently met with. 



Amongst the most ancient 

 and widely distributed of 

 the corals are the Astrcets, 

 so called from the star-like 

 arrangement of the coral- 

 lites. It is remarkable, 

 FIG. 36. Astraa ananus. too, that this genus is one 

 of the most extensively dis- 

 tributed of the modern corals. Astreea rotulosa is 

 abundant in the West Indian seas, while A.favosa 

 is as common in the East Indian waters. An 

 illustration is given of A. ananus (Fig. 36), a very 

 frequent fossil of the Wenlock limestones of the 

 upper Silurian system. In fact, the whole of 

 Wenlock Edge in Shropshire, as Professor Owen 

 first pointed out, is a Palaeozoic coral reef of about 

 thirty miles in length. Here, in the very centre 

 of our land, the haunt of trade and the abode of 

 multitudes, the great sea once rolled and the very 

 dust of the roads was once alive. 



In this interesting and picturesque region of 



