504 The Structure of the Eye of Trilobites. [July, 
The full bibliography of treatises relating to these animals in 
Bronn’s Die Classen und Ordnungen des Thierreichs, carried up 
to 1879 by Gerstacker, contains references to no special paper on 
this subject, and the résumé by Gerstacker of what is known of 
the structure of the eye, only refers to the external anatomy of 
the cornea, the form of the facets and their number in different 
forms of Trilobites. He shows that observers divide them into 
simple and compound ; the former (ocelli) are found in the genus 
Harpes. These “ocelli” are said to be situated near one another, 
and are so large that the group formed by them can be seen with 
the unaided eye; the surface of the single “ocellus” appears, under 
the glass, smooth and shining. From the description and the 
figure of the eye enlarged, from Barrande, it would seem as if 
each eye was composed of three large simple ones; so that these 
eyes are really aggregate, and not comparable with the simple 
eye or ocellus of Limulus and the fossil Merostomata.! Maore- 
over the situation of these so-called ocelli is the same as that of 
the compound eyes of other Trilobites. 
The Trilobites with compound eyes are divided into two 
numerically very dissimilar groups; the first comprising Phacops 
and Dalmanites alone, and the second embracing all the remain- 
ing Trilobites, excepting of course the eyeless genera, Agnostus, 
Dindymene, Ampyx and Dionide. The eyes of Phacops and 
Dalmanites are said by Quendstedt and Barrande not to be com- 
pound eyes in the truest sense, but aggregated eyes (Oculi congre- 
gati). But judging by Barrande’s figures of the eyes of Phacops 
fecundus and P. modestus (Barrande, Vol. 1, Suppl. Pl. 13, Figs. 12 
and 22), and our observations on the exterior of the eye of an 
undetermined species of Phacops, kindly sent us by Mr. J. F. 
Whiteaves, Palzontologist of the Canadian Geological Survey, 
we do not see any essential difference between the form and 
arrangement of the corneal lenses of Phacops and Asaphus, and 
are disposed to believe that the distinctions pointed out by the 
above named authors are artificial. 
For my material Iam mainly indebted to Mr. C. D. Walcott, 
who has so satisfactorily demonstrated the presence in Trilobites 
of jointed cephalo-thoracic appendages. On applying to him for 
specimens, and informing him that I wished to have sections 
1 The eyes of the fossil Merostomata (Eurypterus and Pterygotus) are evidently 
in external form and position, judzing by Mr. Woodward's figure, exactly homolo- 
gous with the ocelli and compound eyes of Limulus. 
