166 On Cyrtoma, a new genus of Fossil Echini da. 



groups, but in tracing the affinities of animals, the naturalist 

 has a duty to perform which requires that attention should 

 be equally paid to those peculiarities of species by which 

 animals of different groups represent each other. Thus 

 the observations of M. Agassiz seem intended to establish 

 a direct affinity between Asterice and Echini, founded on the 

 development of the plates of the testa, but before we can 

 establish an affinity between two groups, it is necessary to 

 trace the peculiarities of species composing them throughout 

 their various phases, from their source in one group to 

 the point to which they lead in another. It is customary 

 to regard Asterice and Echini as closely allied, but no 

 one has yet undertaken an analysis of the subject with a 

 view to point out the particular species in the two groups 

 which approach nearest to each other ; for my own part, 

 the only points of relation I can perceive between Echini 

 and Asterice are, that both are radiated and both confined 

 within a testaceous case, the mechanism of which how- 

 ever is quite different in the two groups. 



In the Asterias, beyond the circular body and five con- 

 centric branches, we find nothing of the radiated struc- 

 ture, particularly in the skeleton. In the first place the 

 form is flat, and the earthy pieces composing the upper 

 and lower parts are quite different from each other in form 

 and function, while those composing the lateral columns of 

 the branches are not radiated but divergent, or at least 

 arranged parallel to the rays; in some the pieces are im- 

 bricated like the scales of serpents, in others nodular 

 like those of Saurions; in all this do the Asterice 

 differ from Echini, every atom of whose skeleton forms 

 a part of a spheroid. M. de Blainville appears to have 

 named the plates composing the lower grooved surface 

 of the Asteriae ambulacral, and the upper plates inter-am- 

 bulacral, and supposes them to agree with those plates 



