II 



STRUCTURE AND DEVELOPMENT OF 

 TRILOBITES 



1. OUTLINE OF A NATURAL CLASSIFICATION 

 OF THE TRILOBITES* 



(PLATE H) 



INTRODUCTION 



WITH the possible exception of the barnacles, no group of 

 arthropods has received more varied treatment by specialists 

 than the trilobites. This taxonomic uncertainty has been 

 due mainly to a lack of knowledge of the structure, and to 

 certain real or fancied resemblances to Limulus. 



The early references of trilobites to the mollusks, insects, 

 and fishes need not be noticed, for since they have been made 

 the subject of special study they have been commonly classed 

 with the Crustacea and placed near the phyllopods by most 

 observers. Quite a number of naturalists, however, still 

 divorce the trilobites and limuloids from the Crustacea and 

 ally them with the arachnids. It is not proposed at this 

 time to discuss the homologies of Limulus, but the trilobites 

 show the clearest evidence of primitive crustacean affinities, 

 in their protonauplius larval form, their hypostoma and 

 metastoma, the five pairs of cephalic appendages, the slender 

 jointed antennules, the biramous character of all the other 

 limbs, and their original phyllopodiform structure. They 

 differ from Limulus, not only in most of these regards, but 



* Amer. Jour. Sci. (4), III, 89-106, 181-207, pi. iii, 1897. 



