On Cyrtoma, a new genus of Fossil Echinida. 1 59 



organs of digestion, consisting of an alimentary canal and 

 stomach; they have also a vascular system consisting of 

 heart, arteries, and veins, together with reproductive organs ; 

 it is highly probable that they are possessed also of a 

 nervous system ; this has not been yet proved, although it 

 has been found in the starfish. Their skeleton consists of an 

 external crust which surrounds the soft parts, and is one of 

 the most beautiful structures with which we are acquainted. 

 In a being so constructed, it is necessary that the skeleton 

 or crust should, on the one hand, be such as to resist external 

 violence, and on the other, to admit of the growth of the 

 soft parts within. A sphere composed of small plates with 

 concentric edges forming radii, would resist external pres- 

 sure while it would be capable of being expanded from 

 within, and would answer the object, but something is 

 required to enable such a structure to stand alone and 

 resist the gravity of its own materials. This is effected 

 in the shell of the Echinus, or sea-egg, by means of the 

 peculiar adjustment of the pieces of which it is composed, 

 so that its mechanism might become as instructive to the 

 engineer in affording new mathematical combination, as the 

 leaves and flowers of certain plants have long since proved 

 to the ancient artists, who first had recourse to them as 

 principles of embellishment in their designs. 



If we take the genus Echinus as the type, we find the 

 shell spheroidal, somewhat flattened at the base, with five 

 pairs of porous bands, like the meridians of a globe, extend- 

 ing between the two poles of the testa, these were thought to 

 resemble the walks in a garden, and hence called ambulacra 

 by the older anatomists. Each pair of ambulacra consists 

 of a double vertical series of small plates placed together, 

 so that the horizontal axis of one plate falls between that of 

 the corresponding ends of the two opposite. Between the 

 different pairs of ambulacra there are broader intervals not 



