36 AFFINITY OF THE TRILOBITES 



of a simple shell of the abdomen, present facts in favour of an analogy existing between the 

 two groups, which ought not to be overlooked, and which place the Trilobites much nearer 

 to this genus, than among the Maloscostraca near the Isopoda. 



SECTION XYI. 



I think I have now shown that the Trilobites can only belong to the first principal 

 group of the Crustacea, or to the Ostracodennata ; the only question being with which of 

 the two orders assumed in this group, it stands in the nearest relation. The reply to 

 this question will follow of itself, when we observe that the Trilobites in an advanced 

 stage of life possessed large eyes, and on that account must have had considerable loco- 

 motive powers, so that beyond all question they must be Aspidostraca or Eittomostraca. This 

 result can be distinctly proved by the following reasons : 



1. All Aspidostraca have compound eyes with a smooth cornea, the Trilobites likewise. 



2. They are frequently covered by large shells, which widely project over the axis of 

 the body ; the Trilobites possess quite an analogous formation of the shell. 



3. These shells consist of two membranous layers, with a thin stratum of the substance 

 situated between them. The lower layer is much more tender than the upper, quite in the 

 same manner as we have found it among the Trilobites. 



4. The Aspidostraca possess tender, soft feet, very easily injured, and such must have 

 been possessed by the Trilobites, to account for their absence in all the fossil remains of the 

 latter. 



5. They are exclusively inhabitants of the water, and only move by swimming ; the 

 habits of the Trilobites must have been similar, because they have no hard organs of motion 

 suitable for crawling. 



6. The Aspidostraca, at least those covered by shells, have usually very small antennae, 

 or none at all, while among the Malacostraca they are very large, and covered by a hard 

 upper membrane. This explains immediately why we miss these organs in the Trilobites. 



7. The different subdivisions of the Aspidostraca exhibit different numbers of body and 

 caudal rings, and correspond partially in this respect with the numerical proportions of the 

 Trilobites. The fundamental number of the thorax divisions is exactly the same among 

 all PsendocephaJa and Malacostraca, and differs only relatively, according to the greater or 

 lesser number of rings which have become combined in the head. 



It appears unnecessary to seek for any further reasons in proof of the affinity of the 

 Trilobites and Aspidostraca, after having exhibited so many important points of similarity 

 between them ; I therefore now conclude this investigation with a short consideration on the 

 true relations of affinity which probably obtain between the two groups alluded to. 



