PALEONTOLOGY: R. RUEDEMANN 
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trace the vertebrates back to the Xiphosura and Eurypterida. The 
phylogenetic importance of the trilobites, Vsrhich were the dominant ani- 
mal class in the Cambrian era, is therefore assured, and it is for this 
reason that observations of new structures in the trilobites are of greater 
interest than they might otherwise be. 
An organ not heretofore recognized in the trilobites is the median or 
parietal eye on the glabella, and yet the question of its presence or 
absence in the trilobites is of considerable phylogenetic importance. 
Beecher, through whose observations we have learned so much of the 
ventral anatomy of the trilobites, would ally them closely with the 
Phyllopods (Branchiopoda) and Walcott, who has made us acquainted 
with an unexpected wealth of Cambrian trilobites and other crustaceans, 
would directly derive them from primitive Branchiopods correlated for 
convenience with Apus-like forms. On the other hand we find authors 
such as Kingsley, Bernard and Jaekel, who advance arguments to prove 
that the trilobites can be traced back directly to arthropods more primi- 
tive than any crustaceans, as for example, the annehds. Bernard would 
even not place them in the class Crustacea. 
In these discussions the absence of median or parietal eyes in the 
trilobites has been emphasized as a feature distinguishing them from 
most primitive crustaceans, notably the Phyllopoda, as well as from the 
Merostomes. It is therefore of interest that the presence of such 
median eyes can be demonstrated in this group. 
The median eye appears, in the majority of cases, as a single tubercle 
upon the glabella. This tubercle has so far been recognized in upward 
of thirty genera, among them all genera of the Asaphidae and repre- 
sentatives of most other families of the trilobites. In studying the 
structure of the tubercle it was found that the median eye presents all 
stages of development seen in other crustaceans, from mere transparent 
thinner spots of the test to a lenticular body covered by a thin cornea. 
The lenticular body is frequently recognized by the pit in the interior 
cast of the tubercle; it was seen in sections oiCryptolithus {Trinucleus) 
tesselatus from the Trenton limestone of New York. Here it appears 
as a glossy, pearly body in the interior cast of the eye tubercle when the 
thin cornea is removed. In sections it is seen to be composed of the 
same substance as the matrix, but bounded on the under side by a car- 
bonaceous layer; and it is also observed that the test above the lenticular 
body is thinned to one-half its normal thickness, thus forming a thin 
cornea. From the absence of a separate crystalline structure in the 
lens, and the presence of the carbonaceous division Hne, we infer that 
there was no hard chitinous lens that would become a separate center 
