AND DISTRIBUTION OF TRILOBITES. 255 
whole can never be divided into various parts with increasing 
age. Such a division only takes place in the lowest stages of 
organic formation in the filaments of Conferve, Worms and 
Radiata. It certainly is possible that this law may have but 
only apparently experienced an exception owing to the process 
of shedding the skin, to which the Trilobites were subject like 
all other Articulata. The circumstance that we meet with no 
differences in the shape of Battus, although all developed Trilo- 
bites are of so various a shape, is certainly a still more striking 
reason against that assumption ; Battus pisiformis being almost 
the only definite species of the genus. 
From the very first attempt at a separation of the Trilobites 
into genera, the compound eyes, in which respect they likewise 
correspond with all the living Crustacea, have been considered as 
particularly important. It is principally an alleged absence of 
them in many Trilobites, upon which particular stress has been 
laid, so much so, that even the cautious Dalman divided the 
genuine Trilobites into the two divisions of blind Trilobites and 
of those furnished with eyes ; Brongniart indeed had to a certain 
extent already preceded him in this, and many others have fol- 
lowed the example. How far this deficiency of eyes can be 
scientifically demonstrated with respect to the genera Ogygia, 
Paradowides, Brongn.,.Conocephalus and Ellipsocephalus, Zenk., 
will be seen from a more accurate investigation of the eyes in the 
Calymene and Asaphi of Brongniart, which according to gene- 
ral opinion were possessed of the faculty of vision. We find 
numerous species in both genera, in which even the naked eye 
can detect the same structure as in many recent genera, but 
scarcely anywhere so beautiful in the latter, nor formed on so 
large a scale. All the species which Portlock and I unite into 
the two genera Phacops and Phillipsia possess a horny mem- 
brane, divided into large, circular, hexagonal facets, which gene- 
rally projects (Phacops) in the form of a spherical segment, in the 
centre of each facet; a crystalline lens is situated behind each 
facet to which it is united, which, together with the horny mem- 
brane, may be separated from the rock. It is probable that a 
peculiar parabolic transparent substance (glaskirper) was situated 
behind the crystalline lens of each eye, since we sometimes meet 
with hexagonal loops that penetrate into the rock, after having 
broken away the crystalline lenses. (Quenstedt in Wiegmann’s 
Archiv, 1838.) This structure of the eye we find preserved in the 
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