AND DISTRIBUTION OF TRILOBITES. 259 
heads, do not occur. The same may perhaps hold good with 
respect to Arges, Goldf., since the Odontopleure, which are 
nearly allied to it, also want the so-called upper eyelid, causing 
the forehead which has been deprived of its maxillary shield to 
appear like a perfect cephalic shield (O. centrina). 
We have still to consider the genus Cryptolithus; that its 
cephalic shield is perfect is beyond all doubt, even if the facial 
suture had not yet been discovered in it; this suture however 
really has been detected in its normal course in a stone cast 
of Cr. Caractaci in the Berlin Museum, but there is no trace 
of eyes, although the facial suture circumscribes as usual the 
so-called upper eyelid. However, even with regard to this 
genus, I am induced to hope for a future discovery of the eyes 
from a Trilobite found in the Goniatite-limestone of Dillenburg, 
which will be subsequently described; I have named it Phacops 
cryptophthalmus (perhaps the Calymene levis, Minster). It cer- 
tainly is not impossible that there may have existed blind Tri- 
lobites, as we even find living Crustacea deprived of eyes; but 
it is precisely a more accurate investigation of these latter which 
proves that all principles of analogy are against the assumption of 
blind Trilobites. We find, for instance, in all recent crustaceous 
animals compound eyes; only some genera wholly parasitical, be- 
longing to the order of the Isopoda (Bopyrus) and to a division of 
Entomostraca (Lernea, Cecrops, &c.), are blind. All these blind 
species are therefore parasites, and indeed such as never quit the 
place they have once fixed themselves in between the gills of 
Crustacea or fishes; they are always more or less degenerated 
in their form, sometimes so much so as to render the recognition 
of the type of their order a matter of impossibility. The study of 
their development has however likewise proved with respect even 
to these animals, at least with regard to those which differ most 
from the usual type (as for instance Lernea, &c.), that they 
possess an eye as long as, after their liberation from the egg, they 
swim about freely; that they however lose this eye in the me- 
tamorphosis which takes place when they have attached them- 
selyes as parasites. This may probably be the case likewise with 
respect to Bopyrus, a constant parasite and therefore blind wood- 
louse, which has indeed (if such a thing can be conceived) the 
most similarity with the Trilobites among the permanently fixed 
Crustacea. It is also very much deformed. On comparing now 
our supposed blind Trilobites with those degenerated animals, 
