Affinities of Trilobites, 19 



its crustaceous, semilunar cephalothorax bears considerable 

 resemblance to that of certain Trilobites, such as the genera 

 Ogygia, Asaphus, Paradoxides, &c. In Limulus, we find reni- 

 form, compound eyes placed widely apart on the back of the 

 head, and consisting of pecuhar facets. We find, also, an indi- 

 stinct trilobed structure of the superior abdominal shield. But 

 then this is composed of a number of confluent segments, so as 

 to appear of one piece ; and, besides the two ocelli, the large 

 crustaceous feet and cheliform antennae throw Limulus far 

 away from the Trilobites. We must, therefore, compare them 

 with Apus and other Aspidophora ; animals which, in my opi- 

 nion, of all the Entomostraca, appear to come nearest to the 

 Trilobita, Here we have a large clypeiform shell, rounded in 

 front, and posteriorly emarginate, which forms a cephalotho- 

 rax, on the back of which are situated three eyes. Of these 

 the two largest are lunated, and obviously correspond to the 

 eyes of Trilobita, although they are placed proportionally 

 much nearer to each other. It is true they are simple, but so 

 appear to have been the eyes of Bumastus *. The abdomen, 

 divided into many distinct segments, the foliaceous feet, the 

 structure of the front of the cephalothorax, the two rudiment- 

 ary antennae, the large labrum and projecting mandibles, all 

 show the affinity of Apus to the Trilobites, more particularly 

 to Asaphus platycephalus, in a specimen of which from Lake 

 Huron, Mr. C, Stokes has discovered a subquadrate labrum, 

 which only differs from that of Apus, in being anteriorly 

 deeply emarginate, while the latter is truncated. Dr. Buck- 

 land has compared this organ to that of crabs, but decapod 

 Crustacea possess a very different structure, and the thing 

 most like this labrum is to be found among the Xipliosura, or 

 still better, among \hQ Aspidophora of Latreille, of which group 

 this naturalist's genus Prosopistoma ought more particularly 

 to be compared with Trilobites. I am not aware, however, 

 that any trilobite has yet occurred with vestiges of ocelli. 



* The distinction between smooth eyes and granulose eyes does not seem 

 to be of much importance in these animals ; for among the existing family 

 of CymothoidcB we not only see tlie males of some species with eyes and the 

 females without them, but we observe neighbouring genera, such as Eurijdice 

 and Nelocira, the one with granulose eyes like a C.-lymene, and the other 

 with smooth eyes like a Bumastus. 



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