AFFINITY WITH STAR-FISHES. 75 



careful comparison of the living animals will show many 

 points in common : thus the five-rayed ambulacra, on 

 the back of the Amphidotus represent the rays of the 

 Star-fish ; and when we place a large number of species, 

 recent and fossil, under review, the passage from the most 

 branching Star-fish to the roundest Sea Egg may be 

 clearly made out through a beautiful gradation of forms. 

 We shall have occasion, probably, to return to the sub- 

 ject in a subsequent chapter. The family of Echinidce, 

 to which these animals belong, was much richer in forms 

 in the earlier world than it at present appears to be ; 

 and from the great facility with which the hard parts of 

 the shelly integument may be preserved, the remains of 

 these creatures have come down to us in a very perfect 

 state. The study of them, therefore, is quite as interesting 

 to the geologist as to the zoologist. It is of importance 

 to the former to know the habits of the living species, 

 that he may form a judgment on what those of the 

 extinct kinds may have been, and thus arrive at just 

 conclusions on the circumstances under which the 

 rocks and gravels, where their remains are preserved, 

 have been deposited. Of the sub-tribe of Heart Urchins 

 (Spatangacece), very numerous species, many of them of 

 highly curious and elegant forms, exist in the oolite and 

 the chalk, and abound in many tertiary deposits. They 

 all characterize marine strata, and generally indicate 

 shallow parts of the sea. Very few of the kinds now 

 living have been found fossilized, except in deposits 

 which are evidently of a very recent date. Thus in 

 these, as in other races of animals, there have been suc- 

 cessions of species, each marking its own era. 



