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PALEONTOLOGY: R. RUEDEMANN 
of crystallization in fossilization, as in the lateral eyes of the trilobites, 
but one corresponding to that of the parietal eye of other crustaceans, 
and especially of the phyllopods, which is a lens- or pear-shaped sac, 
usually filled with water. The parietal eye of the trilobites is hence 
not at all comparable to the larval ocelli of insects or the parietal eye 
of Limulus, the eur3^terids and arachnids in general, where the chitinous 
integument thickens into an exterior lens, but it agrees well in its struc- 
ture with the median eye of the crustaceans; the thin black layer at the 
base of the lenticular body being derived from the pigment of the retina. 
Whether the lenticular cavity was filled with sea-water or a body fluid 
is not known, but there are indications that some trilobites may have 
possessed a pore on the apex of the tubercle, giving the sea-water access 
to the interior. 
Indirect evidence for the visual function of the tubercle is seen in the 
following facts: 
The eye tubercle is the sole prominence on the otherwise smooth 
glabella in the Asaphidae, Trinucleidae, etc. It must therefore have a 
function requiring prominence for its performance. It is always situ- 
ated at the highest spot of the carapace, either on the apex of the bulg- 
ing frontal lobe of the glabella as in Cryptolithus (Trinucleus) , or where 
the glabella abruptly bends downward on the prominent posterior por- 
tion between the last glabellar lobes (Isotelus and Asaphus). It is, 
further, generally situated between the posterior portions of the lateral 
or compound eyes. This position is explained by the fact that the 
median or parietal eye according to its origin is always nearest to the 
brain and this is, in the phyllopods and most other crustaceans, situated 
in the dorsal region of the head, beneath or between the lateral eyes. 
As in the eurypterids, so also in the trilobites, the median eye is often 
found at the posterior extermity of a distinct crest, extending backward 
a short distance upon the glabella. This crest is probably an analogue 
of the 'eye-line' of the lateral eyes of the primitive trilobites, and at 
least in part, marks the path of the nerve leading to the median eye. 
As in the crustaceans and eurypterids, the median eye tubercle is rela- 
tively largest and most prominent in the earliest growth stages, and in 
the later stages it may entirely disappear, as in Isotelus gigas. It is 
likewise better developed in the more primitive orders of the trilobites; 
and the phylogenetically late families of the highest order, the Proparia, 
viz., the Calymenidae and Phacopidae, seem to have practically lost the 
median eye. The Ordovician and Silurian trilobites show well devel- 
oped median eye tubercles; the Devonian forms lack them; the median 
eye, as in the higher crustaceans, having been either reduced again to 
