278 



A REVIEW OF THE TRILOBITES. 



not the limuli far more nearly related than any one of the ordinary 

 entomostracous orders to the decapods ? The grand distinction is 

 the well-known character of the mouth-organs. But may we not 

 consider the six pairs of oral feet in the one, to represent an earlier 

 typical condition of the six pairs of foot-jaws in the other ? With 

 all their points of difference, at least, a transition from the limuli 

 to the decapods, may certainly be conceived with far less violence 

 to nature, than one between the last-named group and the phyl- 

 lopods or other entomostraca. On this view, a distribution of the 

 crustacean orders may be arrived at, as shewn in the annexed table. 

 A combined vertical and horizontal reading of the table brings 

 out the affinities of these orders in accordance with the principles 

 discussed above. 



1. Decapoda. 



2. Stomapoda. 



I. Malacostraca. 



3. Amphipoda. 

 5. Isopoda. 



4. Loemodipoda. 



6. Xiphosura. 



II. 



7. Trilobita. 







III. Entomostraca. 



8. Phyllopoda. 



9. Lophyropoda. 



11. Cirrhopoda. 



10. Siphonostoma. 



We have yet to consider a few other points of inquiry apper- 

 taining to this portion of our subject. These are embraced in the 

 following questions : — First, were the trilobites inhabitants of littoral 

 or of deep-sea zones ; and secondly, were they of sedentary or of active 

 habits — and if the latter, what were their means of locomotion ? For 

 the satisfactory- determination of these questions, our data are far 

 from complete. Analogy, and the fact of a very general occurrence 

 in ripple-marked shales and other rocks indicative of a littoral origin, 

 would seem to denote a shelving coast-line, or, at least, a moderate 

 depth of water, as the ancient habitat of the trilobite. Trilobites 



