so 



By the Rev. T. A. Preston. 



gERHAPS the commonest of the few fossils, found in this 



of the Members of this Society. 



An echinus, as picked up in a fossil state, is a solid, more or less 

 of a melon shape, with one part flat, and with five double rows of 

 dots radiating from a point at the apex of the rounded part. There 

 will also be two rather large holes, either on the flat surface, or else 

 opposite to one another, one in the flat surface and the other at 

 the apex : and this will generally be all that can be observed about 

 the fossil echinus, which will be either a flint cast of the interior, or 

 if taken direct from the chalk, it will be seen to be a hollow " box," 

 filled with chalk, and besides the lines of dots there will also be 

 observed a number of projecting points or " bosses/'' scattered over 

 the surface, the size of these bosses varying extremely with each 

 species. I shall proceed at once, therefore to describe a living 

 specimen, and one of the species most commonly occurring, for I 

 must premise that my remarks will have to be modified according 

 to the species selected for examination. 



I may take this portion, then, found in a fossil state, as the basis 

 of the structure. It consists of a hollow " box " or envelope, called 

 a corona, at first sight very similar to the hard covering of the crab 

 or lobster. But though it is composed of the same material, it is 

 essentially different in structure. In the case of the lobster, if it is 

 broken, the cracks go in all directions without any definite lines, but 

 in the case of the echinus the cracks will be seen to follow (as a rule) 

 very definite lines ; straight, if horizontal, and straight or zigzag, 

 if the crack runs from pole to pole : in fact the corona of the echinus 

 is made up of a large number of little plates, varying in number 



fl^Sf ne i&h Dour h 0 °d> * s that of the Echinus, or Sea Urchin, and 

 A and as there are fortunately several living representatives of 

 these creatures, it seems to be. a fit subject to bring before the notice 



