APPENDIX. 83 



between the last curved points of the side of the thorax. Owing to these various 

 combinations I would expect in Trilobites phyllopod-like respiratory appendages 

 under the pygidium only, and slender, articulated legs, with lateral bristles under 

 the thorax, so thin and articulated by so narrow a joint as easily to break off 

 without leaving more than a puncture as an indication of their former presence. 

 It is impossible to study carefully the synthetic types without casting a side 

 glance at those natural groups, which, without being strictly synthetic themselves, 

 have nevertheless characters capable of throwing light upon the whole subject. 

 And in this connection I would say a few words of Apus and Limulus. If I 

 remember rightly, Milne Edwards considers the shield of Limulus as a cephalo- 

 thorax in which the function of chewing is devolved upon the legs, while he 

 regards the middle region as an abdomen, and the sword-like tail as an appendage 

 sui generis. In the light of what proceeds I am rather inclined to consider the 

 cephalic shield of Limulus as a buckler homologous to that of the Trilobites, and 

 the middle region as a thorax in which the rings show unquestionably signs of a 

 division into lobes as in Trilobites. The tail would then answer to the pygidium. 

 Apus should be compared with the other Crustacea, upon the same assumption as 

 Limulus. 



L. AGASSIZ." 

 " Rio, on board the * Hassler, 



" Feb. 12th, 1872. 



