I42 NEW YORK STATE MUSEUM 



8 The median eye is borne on a tubercle or mound in the 

 Ordovician and Silurian trilobites, while the tubercle is rarely 

 noticed in the Devonian and in few Cambrian forms. In the 

 Devonian forms, similarly as in many crustaceans and in later 

 growth-stages of some asaphids, the strong development of the 

 lateral eyes may have led to a loss of the parietal eyes. In the 

 Cambrian genera evidence is present to suggest that the parietal 

 eyes consisted only of transparent spots or lenslike thickenings of 

 the exoskeleton, hardly noticeable from the outside. 



9 It is a priori to be inferred that the trilobites should, as 

 primitive crustaceans, have possessed median or parietal eyes. 



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