FOSSIL STAR-FISHES AND SEA-URCHINS. 147 



urchins. Thus in the "fairy loaves," as they are 

 called in the Eastern counties, where they literally 

 abound (the chalk fossils known to geologists as 

 Ananchytes ovata), we see five similar rows of perfora- 

 tions ; and even the somewhat differently fashioned 

 tests of the earliest genus of sea-urchins (Palceechinus), 

 dating from Carboniferous if not from Silurian times, 

 have perforated ambulacral plates, showing that these 

 very ancient animals were then in possession of the 

 hydraulic principle which has been of such inestimable 

 value to their race. The Ananchytes of the Chalk, 

 however, have very small tubercles, and the spines 

 formerly attached to them must have been very small 

 and bristle-like, as is now the case with those of the 

 living Cake-urchin (Bryssus lyrifer), not uncommon in 

 the muddy bottoms of the Kyles of Bute, the Spatan- 

 gus, Amphidotus, and many others. This is not the 

 case with the Cidarids found fossilized in the Chalk 

 with them. The very large knobs or tubercles on 

 the tests of the latter animals (which are especially 

 abundant in tropical seas at the present time) give 

 support to large spines, of a club-shape generally, and 

 often ornamented by various devices. Their ball- 

 and-socket principle of jointing, however, was in use 

 in, and has been ever since, the geological epoch 

 termed the Silurian, when the Echini were probably 

 first introduced. In the Oolitic strata we meet with 

 some of the handsomest specimens of Cidarids, and 

 it is very peculiar that, like the fossil Oolitic corals, 



