20 



W. S. MacLeay on the Structure and 



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 la- 

 teral, posterior, abdominal appendages. Excepting Bopyrus^ 

 and certain Lcemodipoda, all the Amphipoda possess these anal 

 appendages, which are generally styliform, articulated and in 

 number two. The LcEmodipoda, however, want these append- 

 ages, because the whole abdomen in them has become eva- 

 nescent, a case totally different from that of Trilobites, which, 

 like Bopyrus, have a well-developed abdomen consisting of 

 many segments. I therefore consider this deficiency of anal 

 appendages 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 affinities 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. 



Decapoda, Lat. 



PoDOPHTHALMA, Leach. 



Animals having their 

 eyes supported on move- 

 able peduncles. 



Stomatopoda, Lat. 



Antenniferous region 

 of head confluent 

 with the thorax. 



Antenniferous region 

 of head distinct 

 from the thorax. 



* Bopyrus may possibly belong to the Trilobita, but I confess I do not 

 see how Agnostus can. Nor do I believe that the latter fossil has any con- 

 nexion with the Annulosa at all. 



