FOSSIL STAR- FISHES AND SEA-URCHINS. 149 



pieces. I have seen silicified specimens of Echini 

 in Chalk flints near Norwich which have had these 

 teeth fossilized, but such examples are exceedingly- 

 rare. Nevertheless, it affords another instance of the 

 persistency of a plan. Generally speaking, the larger 

 number of the Echinoidea of the Chalk seas had 

 the mouth and anal aperture at the base ; and such 

 genera as Ananchytes, Holaster, Micraster, Galerites, 

 etc., are grouped according to the position of these 

 apertures, which is always constant in the same 



Fig. 121,—Cidaris corotiata, showing mode of attachment of the club-shaped spines. 



species. In the recent Echinus, as well as in the 

 fossil Cidarids, the mouth is at the base and the anal 

 orifice at the summit. 



The modern Bryssus (as I have already noted) 

 buries itself in very fine mud, on the organic matter of 

 which it appears to feed, just as earthworms do on 

 the black soils. The Micrasters and Spatangi of 

 the Cretaceous period, which approach the Bryssus 

 very nearly, both in shape and structure, may have 



