412 THE AMERICAN NATURALIST [Vol. LIV 



axial portion as the ancestor. Only a few specialized 

 trilobites like the Kemopleuridae and one subfamily of the 

 Cheiruridae show extensive reduction of the pleural lobes. 



A study of the ontogeny of trilobites with both large 

 and small pygidial shields shows that the pygidia are 

 proportionally larger in the protaspis than in the adult, 

 and the more ancient forms pass through a stage in which 

 they are practically isopygous. This suggests that the 

 ancestor had subequal cephalic and abdominal shields. 

 Since the thorax grows by the breaking down of the pygi- 

 dium, the ancestor should lack free thoracic segments. 

 Curiously enough, among Walcott's remarkable finds in 

 the Middle Cambrian there is an isopygous crustacean 

 without free thoracic segments. It was named Naraoia 

 and referred to the Branchiopoda by Walcott, 6 but since 

 it satisfies the theoretical considerations and evidently 

 can not be referred properly to any other subclass, I am 

 inclined to look upon it as the simplest of all trilobites. 

 The specimens so far described are not fully preserved, 

 but the appendages are apparently biramous and trilo- 

 bite-like, and there are at least three pairs on the head 

 and fourteen on the pygidium. From Agnostidae with 

 subequal shields and two thoracic segments to Naraoia 

 with subequal shields and no thoracic segments is but a 

 step. If the pygidium were built up by the coalescence 

 of once free segments, that step would be in the direction 

 of specialization, but since the reverse is the case, the 

 extraordinary conclusion is reached that the simplest 

 trilobite did not look at all like the traditional benthonic 

 round annelid. 



The study of the ontogeny of many species of trilobites 

 long ago established the fact that in the ontogeny the 

 eyes, which may be entirely absent in the very young of 

 the simplest oculiferous species, appear first on the an- 

 terior margin and during growth move backward on the 

 head. A point which has not been noted is that this 

 movement is correlated with a backward movement of 



e Smithsonian Aliscl. Coll., Vol. 57, No. 6, 1912, p. 175. 



