254 CLARKE. [ VoL. II. 
have been so perfectly retained as to allow of enumeration. 
These specimens have been derived from the shales and lime- 
stones of the Hamilton group at various localities in Western 
New York, those best adapted for the purpose of sectioning 
being from the basal limestones near Centerfield, Ontario 
County. 
THE CHARACTER OF THE VISUAL AREA in the trilobites is 
twofold; (a) it may be covered by a smooth, continuous epi- 
thelial film or cornea, through which the lenses of the ommatidia 
are visible by translucence, and (6) the cornea may be transected 
by the protrusion of the sclera! and limited to the surfaces of the 
ommatidia. To the first group belong species of the genera 
Asaphus, Ilenus, Calymene,* Homalonotus, Proétus, Cyphaspis, 
Acidaspis, Lichas, and others; to the second, the single exten- 
sive family, Phacopide, with its genera, Phacops and Dalma- 
nites (? Harpes,; vide conclusion). The first group may be des- 
ignated by the term /olochroal ; the second group by the term 
Schizochroal, 
PHACOPS RANA is one of the most abundant and characteristic 
species of the Hamilton faunas. Though widely distributed in 
the formations of this age throughout the United States and 
Canada, it is not known with certainty to have been present in 
faunas older than those of the Hamilton, and it does not appear 
to have continued its existence after the displacement of the 
Hamilton faunas. A detailed and very complete description of 
the species, accompanied by copious illustration, is given in the 
1] have found the term ommatidium, proposed by Carriére for the little eyes or 
ocular elements in the compound eyes of Arthropods and Mollusks, a very convenient 
and significant term, but may use it with a little license, as I do not regard the eyes 
included in the second of the above groups as properly compound. ‘The term 
sclera as here used may be open to some objection. It is applied to the interstitial 
test between the ommatidia, and is preferable to the expression cornée opaque of Bar- 
rande. 
2 Professor Edward Orton, of Columbus, Ohio, has allowed me to examine a very 
young individual of Calymene senaria in which the lenses are relatively very large, 
and are strongly suggestive of the character of the lenses in Piacops, although in the 
adult of this species they are so small as scarcely to be detected. This specimen 
suggests the query whether in the holochroal eyes the lenses may, with the advanc- 
ing growth of the animal, become apparently smaller, from close juxtaposition or 
other cause, and also indicates the possibility that the difference in the holo- and 
schizochroal eyes is not as great as it now appears. 
