TO THE EXISTING ARTICULATA. 37 



SECTION XVII. 



At tlic commencement of this discussion, and as expressing the view whicli I shall 

 endeavour to prove, I make the following statement : 



That the Trilobites do not belong to any of the still living families of Crustacea, but 

 represent a distinct group most nearly related to the Aqndosiraca ; that their organization, 

 however, exhibits peculiarities, which at the present day do not occur together in one 

 family, but are dispersed in several heterogeneous groups ; thus, although we have proved 

 in the preceding paragraph that the Trilobites correspond in many essential points of 

 organization with the Aspidostraca, and are not related to any of the still living groups of 

 Crustacea, yet we must not neglect to observe that various important and even typical 

 differences take place between Aspidostraca and Trilohites. These differences consist prin- 

 cipally in the numerical proportions of the thoracic rings, since although the latter certainly 

 vary among the Asjndostraca, they may yet be reduced to several constant fundamental 

 numbers (6, 9, and 1 2) ; wliilst the Trilobites only exhibit a constant number of rings 

 for each separate genus, and the total number cannot be reduced to certain, unchange- 

 able, fundamental numbers or numerical types. In attempting to ascertain with certainty 

 the number of thoracic rings, we certainly meet with the obstacle that we do not know, 

 nor ever can know, the position of the sexual openings among the Trilobites, which position 

 alone indicates with certainty the boundary of the thorax. But even if we exclude for the 

 present the Oleneides with their many- articulated body, and in which the capacity of 

 doubling themselves up is wanting, (since there is the greatest probability that in them the 

 sexual opening was not situated at the last ring before the caudal shield, but on a pre- 

 ceding ring,) yet in the other genera we have the constant numbers 6, 8, 9, 10, 11, and 13, 

 which cannot be reduced to a common fundamental formula. If, therefore, we do not assume 

 that the sexual opening in these genera was also situated at a certain ring of the body 

 before the caudal shield, — say for example's sake at the sixth (2 x 3), or ninth (3 x 3), — 

 we find ourselves obliged to adopt the view that the Trilobites, in point of the fundamental 

 numbers of their thoracic rings, are not constructed according to the law which we have 

 discovered to obtain amongst all Crustacea of the present world. 



This is a most important result, and it perfectly confirms the opinion which I have 

 already pronounced several times, that the ancient types of organization do not correspond 

 with the existing ones, but that they more or less deviate from the plan of the present 

 creation. 



Cuvier, indeed, has acknowledged the truth of this principle, but he did not carry it 

 out ; it has subsequently been often touched upon, especially when speaking of extinct 

 amphibious animals, but, so far as I am aware, it has not even yet been fully recognized by 

 any naturalist. The consideration of this subject is, notwithstanding, the means by which we 

 should be able to show most distinctly that the organized beings of our earth were originally 

 created according to one uniform plan, but that the nature of this plan with regard to the 

 different types, was at first by no means so clearly and distinctly established as it appears to 

 us now in the present representative species. The earlier types, in fact, seem to present 

 the various peculiarities of several gi'oups passing into one another, resulting in forms 



