LARVAL STAGES OF TRILOBITES 185 



on deductions based upon a study of this type. The recent 

 discoveries of the antennse and the exact details of trilobite 

 structure, together with the larval homologies here made and 

 the concordance of trilobites with the theoretical original 

 crustacean, leave almost no doubt as to their true crustacean 

 affinities. Woodward, 37 from another point of view, reaches 

 the same opinion by saying : " The trilobita, being certainly 

 amongst the earliest forms of Crustacea with which we are 

 acquainted, cannot be removed from that class without 

 destroying its ancestral record." 



Restoration of the Protaspis. 



At first thought the attempt to reconstruct the ventral side 

 of the trilobite protaspis may seem a little hazardous or pre- 

 mature, but a careful consideration of all the data leads the 

 writer to undertake this with some confidence. 



The genus Triarthrus is taken for the basis of this restora- 

 tion, as it is to-day the best known of all the trilobites, and 

 its ventral structure has been ascertained to a degree of per- 

 fection of detail which compares favorably with many of the 

 recent crustaceans. 6 ' 7i 8> 9 The writer has studied the structure 

 of many adult and immature specimens, some of them not more 

 than 5 mm. in length, so that fortunately the appendages are 

 known at many stages of growth. Especially are the young 

 and rudimentary limbs near the extremity of the pygidium in 

 adolescent individuals of considerable morphological inter- 

 est, for they agree closely with the phyllopodiform trunk 

 appendages in the metanauplius of Apus, and protozoea of 

 Euphausia, or, in a general way, with the still more rudi- 

 mentary trunk limbs in the nauplius stages of these and other 

 forms. 



It has been definitely ascertained that the cephalon in trilo- 

 bites bears five pairs of jointed appendages or limbs. 9 In lar- 

 val or immature specimens, and in adults in which the glabella 

 retains its primitive structure, this number is indicated on the 

 dorsal shield by the five lobes or annulations of the glabella, 



