26 Acephala. 



they existed alone, gave to the acephalous faunas the singu- 

 lar character of want of symmetry in the sides, combined 

 with a very remarkable symmetry before and behind. The 

 facts of detail to which I here refer, are scattered throughout 

 all modern works on palaeontology and geology. If, however, 

 it be objected to me, that, by recapitulating these facts, I have 

 generalised too much, I may remark, that even though some 

 species form exceptions to the rule, the general character and 

 fundamental relations of these great divisions are not less 

 of the nature I have indicated ; then we must not forget 

 that certain fortuitous or obsolete determinations, collected 

 at hap-hazard from books, cannot from any case be taken into 

 consideration in examining the questions with which we are 

 now occupied. 



As we have already seen in the case of the Echinoderms, 

 the Acephala likewise present very marked modifications in 

 their representatives, from one formation to another ; and, 

 notwithstanding assertions to the contrary, I here repeat 

 what I have long since afiirmed in regard to fishes and 

 Echinoderms, and which the comparative study of a great 

 number of fossil shells has likewise demonstrated to my 

 satisfaction in reference to the mollusca, namely, that the 

 species, viewed in the mass, differ from one geological epoch 

 to another, in the restricted limits of the subdivisions of our 

 great geological formations. No one has hitherto brought 

 forward this result in a more general manner in regard to 

 the mollusca of the cretaceous and Jurassic epochs, than M. 

 D'Orbigny, in his French palaeontology. On my own part, I 

 have pointed out results in every respect similar, in my cri- 

 tical studies on fossil molluscs. Even before that time Mr 

 Williamson had announced, in a short notice of the fossils in 

 the vicinity of Scarborough, that the species differ completely 

 from one formation to another, in the oolitic series. I am 

 not aware, however, that this fact led Mr Williamson to 

 enter into a critical examination respecting these fossils. 

 But it is above all in the tertiary formations that repeated 

 identities in the different formations have been enumerated 

 in the greatest number. Yet, in a memoir which I published 

 on tertiary shells, the final result of which I had long since 



