PALEONTOLOGIC CONTRIBUTIONS 1 37 



New York, with a distinct, deep pit below the exoskeleton in place 

 of the eye tubercle. Other similar cases could be cited. In this 

 final stage the parietal eye of the trilobite may have been like that 

 of the phyllopods. 



In view of the facts observed in recent crustaceans, we consider 

 it quite possible that the parietal eye persisted in the trilobites much 

 longer than our present evidence would suggest, in the form of a 

 thin, smooth spot in the glabella. 



It is a peculiar phenomenon that the most important Cambrian 

 families, the Conocoryphidae, Mesonacidae, Paradoxidae and 

 Olenidae, do not exhibit a trace of this median tubercle, while in 

 these it should be expected above all. We were at a loss to explain 

 this fact until we observed in several beautifully preserved growth- 

 stages of Elliptocephala asaphoides from the Ford 

 collection, that the bulging frontal lobe of the glabella possesses two 

 circular transparent spots, appearing whitish in the black test with 

 a smaller white speck between them, exactly where the young of 

 Limulus p.e. shows its ocelli, and we have here also noted two like 

 faint, though distinctly outlined, spots in the test of specimens of 

 Paradoxides, as P. spinosus. While this matter needs fur- 

 ther study with larger and first-class material, we feel justified in 

 expressing the belief that these early Cambrian genera had their 

 median ocelli, like primitive crustaceans, only in form of trans- 

 parent spots of the test, which in the fossil material are only 

 noticeable in perfectly preserved and but little carbonized 

 exoskeletons. 



7 It is a distinct fact pointing to the visual function of the 

 median tubercle that the genera usually considered as blind because 

 of reduced or absent lateral eyes, are apt to show these tubercles 

 most distinctly, as notably Cryptolithus (Trinucleus) and the 

 related Dionide and Dindymene, and also Agnostus, Eodiscus and 

 Ampyx. On the other hand, it is apparent that the Phacopid genera 

 Phacops and Dalmanites with their highly developed, large, lateral 

 eyes, show the least trace of median eyes. In the Asaphidae, which 

 have also well-developed lateral eyes and retain the median eye, it 

 is seen that the latter retains its usefulness on account of the 

 vertical position of the visual areas of the lateral eyes that do not 

 allow an upward sweep of the vision. 



8 Quite significant as indicating an important function of the 

 tubercle is the fact that it is a constant feature in a great number of 

 genera and species, with otherwise smooth carapaces. We can 

 demonstrate the existence of this tubercle in some thirty odd 



