PALEONTOLOGIC CONTRIBUTIONS 141 



he states (p. 139) : " There is no reason to doubt that the trioculate 

 median eye of decapods, copepods, trilobites and merostomes, in 

 structure and development is essentially like that of Limulus, 

 scorpions, spiders, Apus and Branchipus," and again (p. 148) : 

 " The parietal eye of vertebrates is homologous with the parietal 

 eye of such arthropods as Limulus, scorpions, spiders, phyllopods, 

 copepods, trilobites and merostomes, but not with the frontal 

 stemmata or other ocelli of insects." 



When zoologists of standing postulate the presence of median 

 eyes in the trilobites, paleontologists might as well make a serious 

 search for this important organ of crustaceans and adopt the view 

 that normally the trilobites were provided with a distinct parietal 

 eye. 



Summary 



It is claimed in this paper that most, if not all,, trilobites possessed 

 a median or parietal eye on the glabella. In proof of this assertion 

 the following facts are stated : 



1 A great number of species, belonging to more than thirty 

 genera, possess a distinct tubercle on the glabella. This tubercle 

 occurs alone in many genera, otherwise smooth, as in the Asaphidae, 

 and is hence of functional importance. 



2 In certain cases, as in Cryptolithustesselatus, 

 distinct lenticular bodies were recognized; in others, as in 

 Asaph us expansus, only a thinner, probably transparent 

 test. Many other species show a distinct pit in interior casts of the 

 tubercle, indicating, a lenslike thickening of the top of the tubercle. 

 The median eye therefore probably possessed all the different 

 stages of development seen in other crustaceans. 



3 As in the parietal eyes of the crustaceans and the eurypterids, 

 the tubercles are most prominent and distinct in the earlier growth- 

 stages, notably so in I s o t e 1 u s g i g a s . 



4 The tubercle is especially well developed in the so-called blind 

 forms, where the lateral eyes are abortive, as in Cryptolithus 

 (Triarthrus), Dionide, Ampyx. 



5 The tubercles always appear on the apex on the highest part 

 of the glabella, where their visual function would be most useful. 



6 The tubercle is generally situated between the lateral eyes, like 

 the parietal eye in crustaceans and eurypterids, on account of its 

 close connection with the brain. 



7 Frequently it forms the posterior termination of a short crest, 

 also as in certain eurypterids (Stylonurus) indicating the direction 

 of the nerve. 



