I36 NEW YORK STATE MUSEUM 



similar in other trilobites, as Cryptolithus (Trinucleus) 

 tesselatus, where Beecher's figure of the protaspis shows a 

 relatively large tubercle compared with that of the mature stage. 



6 To the early presence of the tubercle in the individual develop- 

 ment of the trilobites just described, corresponds its prevalence in 

 the mature stages of the phylogenetically older and more primitive 

 genera, as far as the differential development of the lateral eyes 

 and the correlated reduction of the median eye has not obliterated 

 the former relation between development of race and median eye. 

 We thus find the median eye much better developed in the genera 

 of the earlier and more primitive orders of the Hypoparia and 

 Opisthoparia than in the third order of trilobites, the Proparia, and 

 the most advanced and phylogenetically and historically late 

 families of this order, the Calymmenidae and Phacopidae, seem to 

 have practically lost the eye tubercle, if not altogether the median 

 eye (see below, p. 137). Expressed in terms of geologic history, we 

 may say that the Ordovician and Silurian trilobites prevailingly 

 possess the parietal eye tubercle while the Devonian forms nearly 

 all lack it. 



Also within certain families the same relation of tubercle to 

 phylogenetic age can be observed, as notably in the Asaphidae, 

 where the phylogenetically older genera, as Asaphus, Onchometopus, 

 Basilicus, Asaphellus, Symphysurus and Nileus, retain the tubercle 

 through life, while the younger and more advanced genera, as 

 Isotelus and Megalaspis, lose it in the mature stageas tubercle. 



The case of the gradual disappearance of the eye tubercle in the 

 trilobites is parallel to that in the crustaceans in general, where the 

 highest subclass, the Malacostraca, lacks the median eye as an 

 exterior organ while it prevails in the lowest subclass, the Branchi- 

 opoda. 



It is to be remarked in regard to the later genera which lack the 

 median eye tubercle, that we are not yet at all sure that they also 

 lack the median eye, for as the median eye wanders inward in the 

 crustaceans and later vertebrates, so it may have done in the later 

 trilobites. Indeed we have direct evidence for this supposition in 

 the gradual disappearance of the eye tubercle in Isotelus 

 g i g a s , leaving but a smooth spot in its place, and in some species 

 where the exoskeleton reveals no longer any trace of a parietal eye, 

 the interior cast exhibits a distinct pit, showing that the organ was 

 still present on the inside of the exoskeleton. We figure in illustra- 

 tion of this (see pi. 34, fig. 9) a glabella of Cheirurus 

 niagarensis (Hall) from the Silurian Rochester shale of 



