96 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
marked, inasmuch as the perpendicular furrows are arranged in a shallow crescentic depression; and impres- 
sion d shows besides the obscure furrows a number of irregularly rounded depressions. Larger similar ones 
occur at f, and in part extend over toward g. 
The meaning of these impressions, or their myologic significance, could be discussed, although such 
discussion might rather be termed guessing. 
Inner organs, such as the heart and stomach, might have been attached to the shell along impressions a 
and b. Also along or behind c and h, which both continue into the free cheeks, ligaments or muscular 
fibers may have been inserted. From d, e, f, and g, muscles have very likely gone out to the cephalic 
appendages. Against this it may be urged that impression d is too far forward to have belonged to the 
first pair of feet. Again, the impression h may in reality represent two confluent muscular insertions, from 
the first of which, in that case, arose the muscles of the fourth pair of cephalic feet. Were this the case, the 
muscles of the first pair of cheek feet should be attached at e. And d in turn may be explained as the 
attachment of the muscles of the antennae, k those of the hypostoma, and from i possibly those of the epistoma. 
That k is here named as the starting point of the hypostomial muscles and not those of the antennae, depends 
partly on the analogous position of i and partly on the fact that the hypostoma of Nileus armadillo (text 
figure, in which the outline of the hypostoma is dotted), by reason of its wing-like border, could not have 
permitted the antenna to reach forward, but rather only outward or backward. 
My own explanation would be that impressions e, f, and g correspond to the glabellar 
furrows, h the neck furrow, and all four show the places of attachment of the appendifers. 
Those at d may possibly be connected with the antennas, although I should expect those 
organs to be attached under the dorsal furrows at the sides of the hypostoma. It will 
be noted that either b, k, or i correspond well with the macula; of the hypostoma and some 
or all of them may be the points of attachment of hypostomial muscles. They correspond 
also with the anterior scars of Dalmanites. 
Eyes. 
While I have nothing to add to what has been written about the eyes of trilobites, this 
sketch of the anatomy would be incomplete without some reference to the little which has 
been done on the structure of these organs. 
Quenstedt (1837, p. 339) appears to have been the first to compare the eyes of trilo- 
bites with those of other Crustacea. Johannes Muller had pointed out in 1829 (Meckel's 
Archiv) that two kinds of eyes were found in the latter group, compound eyes with a smooth 
cornea, and compound eyes with a facetted coat. Quenstedt cited Trilobites esmarkii Schlo- 
theim (= Illcenus crassicauda Dalman) as an example of the first group, and Calymene ma- 
crophthalma Brongniart (= Phacops latif r 011s Br onn) for the second. Misreading the some- 
what careless style of Quenstedt, Barrande (1852, p. 133) reverses these, one of the few 
slips to be found in the voluminous writings of that remarkable savant. 
Burmeister (1843; 1846, p. 19) considered the two kinds of eyes as essentially the 
same, and accounted for the conspicuous lenses of Phacops on the supposition that the cornea 
was thinner in that genus than in the trilobites with smooth eyes. 
Barrande (1852, p. 135) recognized three types of eyes in trilobites, adding to Quen- 
stedt's smooth and facetted compound eyes the groups of simple eyes found in Harpes. In 
his sections of 1852, pi. 3, figs. 15-25, which are evidently diagrammatic, he shows sepa- 
rated biconvex lenses in both types of compound eyes, Phacops and Dalmanites on one hand, 
and Asaphus, Goldius, Acidaspis, and Cyclopyge on the other. Clarke (1888), Exner (1891) 
and especially Lindstroem (1901) have since published much more accurate figures and 
descriptions. The first person to study the eye in thin section seems to have been Packard 
(1880), who published some very sketchy figures of specimens loaned him by Walcott. He 
