24 Acfiphala. 



cies of which already ahound in the Jurassic formations. 

 The whole family of Spatangi, that is to say, the family 

 which approaches nearest the Holothuriae, does not go beyond 

 the cretaceous formations. The plates and spines of the 

 coal formation which have been assigned to Cidarites, do 

 not belong to this family ; they are the remains of particular 

 genera of Crinoides covered with spines. Yet, in our zoolo- 

 gical systems, all these types are placed upon the same level, 

 and if they are arranged one above another, it is without any 

 anxiety about the analogy which exists between their gra- 

 dation and the order of succession in which they appear in 

 the series of formations. So much is this the case, that 

 what M. de Humboldt says, in such a picturesque manner, in 

 his Kosmos, of the aspect of the sky which presents to us 

 every evening, as a real image, the assemblage of celestial 

 bodies, many of which have ceased to exist for myriads of 

 years, may be applied with equal truth to the idea generally 

 given to us by the frameworks of our zoological systems, which 

 likewise hold up to us these witnesses of bygone times as 

 existing realities. v 



The Acephala afford us a not less striking example of these 

 relations between the organic characters of a well character- 

 ised zoological group, and the time of the appearance of its dif- 

 ferent types. In order to shew this connection more distinctly, 

 I may be permitted to premise a few general observations on 

 this class. Mr Owen was the first to shew that the Brachio- 

 pods ought not to be regarded as a separate class, but that 

 they may be conveniently arranged on the same line with 

 the Monomyaires and the Dimyaires. To prove this asser- 

 tion by new arguments, I have only to bring to mind that 

 these fundamental sections of the class of Acephala are closely 

 allied to each other by the connection of their principal forms, 

 and by their respective position in the midst of the ambient 

 elements, as I have shewn in my memoir, Sur Us moules de 

 Mollusques vivans et fossiles, to which I refer. I shall here 

 merely state that the Brachiopods exhibit an inverse sym- 

 metry when compared with that of the regular Dimyaires. 

 In the former, the right and left sides are of very different 

 conformation, and the animal is constantly lying on one of 



