256 EMMERICH ON THE MORPHOLOGY, CLASSIFICATION 
most various rocks, even in such as are very unfavourable to the 
preservation of animal remains, and frequently in surprising 
beauty. Other Trilobites, indeed numerous species, as Asaphus 
expansus and Illenus crassicauda, Dalm., and their allies, possess 
a very different structure of the compound eye from the one just 
described. The horny membrane in them is perfectly smooth 
and lustrous, the external upper surface being not at all facetted ; 
instead of this we observe, in well-preserved limestone fossils, spe- 
cimens possessing a shining and transparent horny membrane, 
and beneath the latter an extremely fine net, formed of hexagonal 
meshes. A transversal section of the thin crust exhibits striz 
directed perpendicularly to the smooth horny membrane, which 
are formed by the lateral boundaries of the extremely numerous 
crystalline cones of the respective eyes. I am not prepared to 
decide whether the fine reticular delineation has only been pro- 
duced by the facetted nature of the internal surface of the horny 
membrane, or, as in Branchipus, by a reticulated membrane. As 
a matter of course, so tender a structure, which can only be ob- 
served by the microscope, can be distinctly visible but under 
particularly favourable circumstances ; indeed it has only been 
detected in well-preserved specimens of the above-named Trilo- 
bites and their allies, but by no means in all Trilobites with a 
horny membrane. [See Quenstedt’s above-mentioned treatise 
‘On the Structure of the Eyes of Trilobites.’] Finally, we have 
still to adduce a third difference in some species of Calymene, 
viz. in C. Blumenbachii and Tristani; and also in Homalonotus. 
The first were with general consent united into one genus 
with the large net-eyed species, although no one has as yet suc- 
ceeded, as far as I am aware, in observing the structure of the 
eyes in them. Instead of eyes there is at the locality where we 
should expect to find them a cavity. I have termed these eyes, in 
the absence of a more suitable expression, and after the example 
of Dalman, Oculi hiantes. The eye being certainly never concave 
during the lifetime of the animal, the hole has most probably 
only been produced after death by the destruction of the eye itself, 
since it occupies the exact locality where the eyes are wanting 
in all the other Trilobites. It is most probable that the cause 
of this easy destructibility of the eyes may arise from the nature 
of the substance of the horny membrane ; a decay of the horny 
membrane must naturally produce an annihilation of the whole 
of the eye. Now on comparing the eyes just described with the 
