NATURAL CLASSIFICATION OF THE TRILOBITES 123 



Taking the early protaspis stages in Sohnopleura, Lios- 

 iracuK, or Ptyclioparia, it is found that they agree exactly 

 ^yith the foregoing diagnosis in its most elementary sense. 

 Since they are the characters shared in common by all the 

 larvEe at this stage, they are taken as primitive and accorded 

 that value in dealing with adult forms possessing homolo- 

 gous features. Therefore any trilohite with a large elongate 

 cephalon, ej-es rudimentary or absent, free-cheeks ventral 

 or marginal, and glabella long, cylindrical, and with five 

 annulations, would naturally be placed near the beginning of 

 anj- genetic series or as belonging to a very primitive stock. 



Next must be considered the progressive addition of char- 

 acters during the geological history of the protaspis, and in 

 the ontogeny of the individual during its growth from the 

 larval to the mature condition. It was shown in the paper 

 already referred to, that there was an exact correlation to Ije 

 made between the geological and zoological succession of 

 first larval stages and adult forms, and therefore both may 

 be reviewed together. 



The first important structures not especially noticeable in 

 all stages of the protaspis are the free-cheeks, which usually 

 manifest themselves in the meta- or para-protaspis stages, 

 though sometimes even later. Since they bear the visual 

 areas of the eyes, when they are present, their appearance 

 on the dorsal shield is practically simultaneous with these 

 organs, and before the eyes have travelled over the margin 

 the free-cheeks must be wholly ventral in position. When 

 first discernible they are very narrow, and in Ptychoparia and 

 Sao include the genal angles. In Dalmanites and Cheirurus, 

 however, the genal angles are borne on the fixed-cheeks. If, 

 as Bernard '^ concludes, the crustacean head has been formed 

 by the bending under, to the ventral side, of the anterior 

 segments of an ancestral carnivorous annelid, this furnishes 

 a means of further determining and also of satisfactorily 

 correlating the prime significance and importance of the free- 

 cheeks. 



Since the free-checks are ventral in the earliest larval 



