AND DISTRIBUTION OF TRILOBITES. 257 
corresponding organs of the above-mentioned four genera that 
were considered as blind, we find that two of them, viz. Ogygia and 
Conocephalus, are allied to Asaphus expansus and A. crassicauda, 
the two others, Paradorides and Ellipsocephalus, are, on the 
other hand, allied to the last-described Trilobites, viz. to C. Blu- 
menbachii. In the former we observe small protuberances at the 
locality where the eyes are situated in other Trilobites, viz. at 
the facial suture, which possess the form of eyes, and are covered 
like them by that part, resembling an eyelid, which adjoins the 
facial suture. These protuberances want only one thing, namely, 
the above-described structure of compound eyes. Owing to 
this last reason the Trilobites belonging to this group have 
been pretty generally considered as blind. Zenker indeed even 
went so far on this account as to designate their eyes by a par- 
ticular term, he called them winged protuberances. If all the 
other Trilobites possessed eyes furnished with large facets, such 
as we find in Calymene macrophthaima, Asaphus caudatus, A. mu- 
cronatus (among the species of the genus Phacops), some plau- 
sible reasons might be adduced in favour of the assumption 
which denies the function of eyes to these protuberances. It so 
happens, that all Ogygie and Conocephali have hitherto been 
found only in the clay-slate and grauwacke: now a structure 
of eyes like those of Phacops and Phillipsia may certainly be 
preserved in such rocks, as indeed is proved by Phacops ma- 
crophthalmus (Calym.) occurring in the roofing-slate of Wissen- 
bach, by Ph. proavius from the grauwacke of Ginec in Bohemia, 
‘and by the Phillipsia equalis from the clay-slate of Herborn ; 
but how is it possible to expect a preservation of the microsco- 
pically fine structure of the eyes of Asaphus expansus, which can 
only be observed, provided the horny membrane is perfectly 
transparent, in such rocks as grauwacke and the clay-slate? How 
could this be possible in rocks of this nature? Supposing even 
that we had at our disposal not merely impressions in the rock, 
but the crust or shell of the animal itself, the most we could 
expect to find would be a tolerably smooth, but not a transpa- 
rent horny membrane, and yet this condition is absolutely re- 
quisite to enable us to observe the compound structure of eyes 
with a smooth, not facetted horny membrane. It is by no means 
uncommon to find not the least perceptible structure of the eyes in 
allies of the A. expansus from limestone rocks, which are highly 
favourable to the preservation of the hard parts of the animal ; 
