No. 2.] EVES OF ARTHROPODS. 265 
Barrande, 1852 (Systéme stlurien du centre Bohéme, Vol. L., p. 
185), recognizes three distinct types of structure, two of which 
are similar in breadth to those of Quenstedt, though differently 
interpreted. The third type of structure is exemplified in the 
genus Hares, the eye of which is considered as an aggregation 
of ocelli (two or three in each eye). For the schizochroal eyes 
Barrande establishes the fact that the sclera (cornée opaque) of 
the visual surface is identical with that of the test of the head, 
and continuous with it. He also regards the existence of a 
transparent cornea covering the entire surface as suggested by 
Burmeister, as probable, but admits that he has been unable to 
assure himself upon this point. For the holochroal eyes the en- 
velope of the visual surface is shown to be of different character 
than that of the test, and consists of a general cornea covering 
the entire visual area. His observations do not seem to have 
led to a decisive conclusion in regard to the simple or compound 
character of this cornea. 
Packard, 1880 (The Structure of the Eye of Trilobites: Amert- 
can Naturalist, p. 503), quotes a resumé of the status of the dis- 
cussion as given by Gerstiacker in Bronn’s Classen und Ordnungen 
des Thierreichs, which is virtually a reiteration of Barrande’s 
opinions. The “‘ocelli” of Hares are regarded as aggregate 
eyes, not comparable with the simple eyes or ocelli of Lzmulus 
and the Merostomata. The eyes of Barrande’s first and second 
groups are considered as true compound eyes and not aggre- 
gated eyes; no essential difference is recognized in the form 
and arrangement of the corneal lenses of Phacops and Asaphus, 
and the distinctions pointed out by Quenstedt and Barrande are 
considered artificial. 
The sections used by Professor Packard in the comparative 
study of the trilobite eye appear to have been altogether of 
holochroal forms. A close correspondence in structure is de- 
monstrated between the eye of Asaphus and that of Limulus. 
In the Lzmulus eye the lenses are covered by a continuous cor- 
neal layer, which does not make itself apparent in Professor 
Packard’s sections of Asaphus, although it undoubtedly exists. 
It may be questioned whether the conclusion drawn from this 
comparative study is not too broad, viz., that the trilobite eye is 
organized on the same plan as that of Lzmudlus.1 
1 Professor Packard’s observations upon the eye of Asaphus show no indication 
of the existence of interstitial epithelium between the lenses, an extremely important 
