LARVAL STAGES OF TRILOBITES 167 



are chiefly : the elongation of the body through the gradual 

 addition of the free thoracic segments ; the translation of 

 the eyes, when present; the modifications in the axis of the 

 glabella ; the growth of the free-cheeks ; and the final assump- 

 tion of the mature specific characters of pygidium and 

 ornamentation. 



In the present paper the larval stages of several species are 

 described and illustrated for the first time, and a review is 

 undertaken of all the known early larval stages thus far 

 described. This work would have no special interest in 

 itself were it not for the fact that, with our present under- 

 standing of trilobite morphology, it is possible to reach 

 some conclusions of general importance which have a direct 

 bearing on the significance and interpretation of several of 

 the leading features of the trilobite carapace, and incidentally 

 upon the structure and relations of the nauplius of the 

 higher Crustacea. 



The Protcfspis. 



Barrande 3 recognized four orders of development in the 

 trilobites, as follows : 



TYPES 

 ( Head predominating, incomplete. \ 



I. < Thorax nothing or rudimentary. >- Sao hirsuta. 

 ( Pygidium nothing. ) 



( Head distinct, incomplete. ) 



II. } Thorax nothing. [ Trmculeus ornatus and 



( Pygidium distinct, incomplete. ) a11 Agnostus. 



r Head complete. \ 



III. -! Thorax distinct, incomplete. > Arethusina Konineki. 



(. Pygidium distinct, incomplete. ) 



f Head complete. ^ 



IV. < Thorax complete. > Dalmanites Hausmanni. 



( Pygidium distinct, incomplete. ) 



A study of these groups shows at once that they form 

 a progressive series in which the first alone is primitive. 



