264 CLARKE. (VoL. II. 
when normally preserved, but the corneal lenses are hollow, and 
the sclera represented by a thin wall of silica. This condition 
is sometimes modified by the removal of the entire upper sur- 
face of the visual area, generally by its adherence to the outer 
part of the matrix, leaving only vertical tubes representing the 
ommatidial cavities. 
While the foregoing observations and the essential conclu- 
sions therefrom in regard to the structure of this phase of the 
trilobite eye, agree in some respects with the opinions of earlier 
writers upon this topic, there are many important points of dif- 
ference and various features of structure which have not before - 
been noticed. I therefore give a brief historical review of the 
observations upon the subject. 
Quenstedt, 1837 (Wiegmann’s Archiv fiir Naturgeschtchte, 
Vol. I., p. 340), was the first to recognize two distinct types of 
structure in the eyes of trilobite, and divided them into 
1. * Aggregated eyes having a facetted cornea. 
2. Aggregated eyes having a smooth cornea. 
In both groups the cornea was regarded as the direct continu- 
ation of the superficial layer of the test of the cheek. The 
first type of structure was represented by the eye of Phacops 
latifrons, Bronn (z.e., the Phacopidz). The second type was 
exemplified by ///e@nus crassicauda, Dalman (z.e., holochroal 
eyes), in which the facets are said to be in relief upon the 
internal surface of the general corneal (visual) area, each facet 
being formed by a lens or crystalline body, behind which lies a 
vitreous body, penetrating deeply into the organ. In these the 
cornea was regarded as composed of two distinct layers, of 
which the outer is quite smooth, the inner very finely reticulate. 
Burmeister, 1843 (Organization der Trilobiten, p. 19), regarded 
the structure of the holochroal eye as directly comparable in all 
respects to that of the eye in Branchipus stagnalis, and indorsed 
Quenstedt’s view of the compound corneal layer, while admit- 
ting but a single type of structure. He assumes with respect 
to Phacops that the cornea must have been more destructible 
than in the other trilobites, and by its removal the facetted sur- 
face exposed. 
* As I have not had access to this work, I am compelled to take the summary of 
these observations as given by Barrande (Syst. sz/., p. 133). 
