668 



STRUCTURE AND AFFINITIES OF TRILOBITES. 



merits, the foliaceous feet, the structure of the front of the cephalothorax, the two rudimentary 

 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 ante- 

 riorly deeply emarginate, while the latter is truncated. Dr. Buckland 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 Xiphositra, or, still better, among the Aspidophora of La- 

 treille, of which group this naturalist's genus, Prosopistoma, ought more particularly to be com- 

 pared with Trilobites. I am not aware, however, that any Trilobite has yet occurred with vestiges 

 of ocelli. 



Still there are characters which, in my opinion, distinguish Trilobites from almost all other 

 Crustacea ; and among these characters I would particularly mention the absence of all lateral, 

 posterior, abdominal appendages. Excepting Bopyrus 1 and certain Lcemodipoda, all the Amphi- 

 poda possess these anal appendages, which are generally styliform, articulated, and in number two. 

 The Lcemodipoda, however, want these appendages, because the whole abdomen in them has become 

 evanescent, a case totally different from that of Trilobites, which, like Bopyrus, have a well-deve- 

 loped abdomen consisting of many segments. I therefore consider this deficiency of anal ap- 

 pendages to a well-developed abdomen, when joined with the evanescent feet, and the total absence 

 of antennae, to be characters separating the Trilobita from all Crustacea except Bopyrus. The af- 

 finities of the group may be roughly expressed by the following diagram. 



If we allow any accuracy to belong to the foregoing remarks on the affinities of Trilobites, it will 

 follow that the class of Crustacea may for the present be distributed into orders, thus; viz. : 



Normal Group. Orders. 



Podophthalma, Leach. HDecapoda, Lat. Antenniferous region of head confluent with 



Animals having their eyes J the thorax, 



supported on moveable pe- ] Stomatopoda, Lat. Antenniferous region of head distinct from 



duncles. L the thorax. 



and the females without them, but we observe neighbouring genera, such as Eurydice and Nelocira, the one with 

 granulose eyes like a Calymene, and the other with smooth eyes like a Bumastus. 



1 Bopyrus may possibly belong to the Trilobita, but I confess I do not see how Agnostus can. Nor do I be- 

 lieve that the latter fossil has any connexion with the Annulosa at all. 



