On Cyrtoma, a new genus of Fossil Echinida. 167 



known in the Echini by corresponding terms. To esta- 

 blish the affinity between starfish and Echini, M. Agassiz 

 thinks it will be necessary to represent the former as if 

 it were globular instead of flat, and " as if the upper lateral 

 plate of one ray was soldered to the upper part of the next ;" 

 in other words, as if the axis of the upper and lower 

 faces of the testa were altered, and the rays bent down, 

 so that the apex of the upper surface of one should be 

 made to fall into the vertex of the lower surfaces of two, 

 and vice versa. If we were to take the liberty of making 

 so many alterations in animals, it would be a very easy 

 thing to construct affinities ; but we think it the more 

 philosophical course to study nature as we find her. 

 Although my business on the present occasion is strictly 

 with the Echinida, yet in seeking a place in that little 

 known family for our new genus, we are desirous of exa- 

 mining in the first instance how the groups composing 

 it are constructed, and what their true rank and position 

 are, in the animal kingdom. 



Before we place implicit faith in the above view of re- 

 duction of types, let us examine the principle on which it is 

 founded, namely, that the Asterias, Holothuria, and Echinus, 

 are subject to one law of development, and that law M. 

 Agassiz tells us is discovered in the growth of the Echinus. 

 The young Echinus has a small number of plates in each 

 vertical series ; they appear to be slowly increased in size 

 by the deposition of calcareous matter at their circumference 

 until those that surround the mouth have completed their 

 growth. The superior plates continue to shoot out and 

 grow from the top downwards, on the periphery of the 

 body, which remains depressed so long as the inferior 

 are the only plates consolidated; but in proportion as a 

 greater number of plates become immoveable, and there 

 is formed in the upper region a greater number of plates 



