s 



146 C. E Beecher — Larval forms of Trilobites 



lishes the comparisons made by Lang, so that the five cephalic 

 limbs of Trilobites would correspond " to the 5 typical limbs 

 of the Crustacean head, and the Trilobites might then be 

 regarded as original Entomostraca, to be derived from the 

 same racial form as the Phyllopoda " (Lang). Also, the phy- 

 logeny and relations of Eurypterus, Limulus, and the Trilo- 

 bites, are with the Crustacean radicle. The information now 

 at hand admits of still further conclusions regarding the affini- 

 ties of the Trilobita. 



The larvse of Acidaspis and Phaethonides, together with 

 those of Sao, Elliptocephala, and Ptychoparia, indicate that 

 Agnostics is neither the phylotypembryo nor the phylo-phyl- 

 embryo, but is really the adult equivalent to an early seg- 

 mented stage of the higher genera. The characters of the 

 youngest larvse expressed in the non-differentiation of the 

 cephalon and pygidium, the preponderance of the cephalon, 

 carrying at this time all the appendages, some of which are 

 the antennse, are strong indications of the likeness of this stage 

 to the ISTauplius or Meta-nauplius of true Crustacea. JSTo 

 ocelli are known, however. The early segmented stages must 

 have resembled, more or less, such late larval Crustacean forms 

 as the larva of Bopryus, one of the Isopoda, and, as suggested 

 by Woodward and Edwards, the Trilobita may be considered 

 as ancient or proto-isopods. 



The fact that most of the smallest larvse observed belong to 

 the first order of Barrande, and that the second and succeed- 

 ing orders represent, more or less closely, stages of develop- 

 ment of an individual from the first, is strong evidence that 

 the first order, or the unsegmented condition, with dominant 

 cephalon and undefined pygidium, is the primitive stage. It 

 is therefore probable that, with more complete material, the 

 young of Trinucleus, Agnostus (= types of 2d order), Are- 

 thusina Konincki (= 3d order), Dalmanites Ilausmanni, 

 Proetus, etc. (= 4th order), will be found to have a first larval 

 stage like Sao, Ptychoparia, and Acidaspis, thus bringing all 

 into perfect accord. 



Upon this basis, the following classification of the stages of 

 development is suggested : 



[ Cephalon predominating, other parts not 

 xr v 9 J separated from it. Examples: earliest 

 up lus ' 1 known stage of Sao, Ptychoparia, and 

 [ Acidaspis. 

 f Cephalon distinct. 

 | Thorax nothing. 

 Phylembryonic ■{ Pygidium distinct. 



| Examples : 2d stage of Sao, etc., and Bar- 

 (^ rande's larvae of Agnostus and Trinucleus. 



