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NEW YORK STATE MUSEUM 



along this line that the lenticular body is apt to break out when 

 pressure is exerted on the tubercle. In thin sections of the over- 

 lying crust (see pi. 35, fig. 5), we find that the integument thins 

 to less than one-half its normal thickness above the tubercle, 



wherefore the eye tubercle 

 is quite flat and less distinct 

 where the integument ad- 

 heres to the mold (Lor- 

 raine specimens), while it 

 is more prominent in the 

 molds or interior casts. 



t-.. ' n „ * 1 • *. v, * ■ 1 This proves that the tuber- 



Fig. 40 Cryptolithus tesselatus r 



Green. Diagrammatic section through eye c ^ e was n °t a ^ ah an ex- 

 tubercle to show relation of integument: terior spinelike organ, but 

 a (figured on plate 35, figure 5), to the essentially an interior organ 

 underlying lenticular body, b (see plate 35, e ven where it is most dis- 

 figures 3, 4); c, matrix of the interior of ,. , ,, - 

 ;, tmct and was therefore 

 the carapace . 



early observed as in Cryp- 

 tolithus. It agrees with this nature of an organ situated essentially 

 below the integument, that the parietal eye in the Cambrian trilobites 

 is not visible as a tubercle, and that in the later stages, as in 

 Cheirurus niagarensis (see p. 136) it again recedes 

 entirely below the integument. 



From the absence of. a separate crystalline structure in. the lens, 

 and the presence of a carbonaceous division line, we infer that there 

 was no hard chitinous lens that would become a separate center of 

 crystallization in fossilization as in the lateral eyes of the trilobites, 

 but one corresponding to that of the parietal eye of other crusta- 

 ceans, and especially of the phyllopods, which is a lens-shaped or 

 pear-shaped sac, usually filled with sea water. 



In regard to the parietal eye of the phyllopods, Patten (no. 13, 

 p. 144) says : " In the phyllopods, although the parietal eye is 

 often very highly developed, it lies well below the surface, and 

 there is no thickening whatever of the adjacent ectoderm, or of the 

 chiten, to form a lens or vitreous body for them. The frequent 

 absence of a lens or vitreous body, in the otherwise well-developed 

 parietal eye of arthropods, is remarkable since it does not occur 

 in the other types of arthropod ocelli." On page 148 it is added: 

 " The most primitive type of a parietal eye is seen in the nauplii of 

 phyllopods and entohiostraca, where the eye is a pear-shaped sac, 

 opening by a median pore or tube on the outer .surface of the head." 

 This parietal eye is, nevertheless, according to the same author 



