PALEONTOLOGIC CONTRIBUTIONS 



135 



. 5 As the parietal or median eye in the eurypterids and other 

 arthropods, the tubercle is relatively largest and most prominent 

 in the earliest growth-stages and it becomes less and less promi- 

 nent in the later stages, apparently (see below) entirely dis- 

 appearing in many genera. Patten (no. 13, p. 148) says of the 

 parietal eye: "The parietal eye of arthropods is an important 

 visual organ until the lateral eyes, which represent a later product, 

 are fully developed. It may then diminish in size and activity, but 

 it rarely, if ever, wholly disappears." Clarke and Ruedemann (no. 

 5, p. 117) have shown that in the case of all eurypterids where the 



Fig. 41 Isotelus gigas Dekay. A small specimen in the Ogygites 

 stage, showing the large tubercle upon the glabella, x 6. (From Raymond) 



Fig. 42 B a s i 1 i c u s tyr annus ('Murchison). An early asaphid with 

 distinct tubercle. (From Raymond) 



earliest growth-stages could be obtained, the ocellar mound or 

 tumescence exhibits a distinct tendency to both larger size and 

 greater prominence in the earlier growth-stages than in the adult. 

 They add : " There is also evidence that the ocelli themselves are 

 relatively larger." 



The same significant connection between the ontogenetic stage 

 and the relative development of the tubercle is distinctly shown in 

 the trilobites. The best case available here is that of Isotelus 

 gigas, where Raymond (see text fig. 41). has figured and noted 

 the large tubercle on the glabella of the smallest specimen showing 

 the dorsal aspect (3.5 mm long) and pointed out that in this and 

 other features the glabella of the growth-stage is like that of an 

 adult specimen of Basilicus, which is an earlier asaphid (Ray- 

 mond, no. 21, p. 250). We figure here (see plate 34, figure 6) a 

 specimen of Isotelus gigas 40 mm long in which the tubercle 

 is still visible with the naked eye ; the same is true of a specimen 

 65 mm long while in the large specimens found at Trenton Falls 

 nearly a foot long, the tubercle has disappeared altogether. It is 



