Review of Recent Geological Literature. 
147 
lenses of the eye as arranged in oblique rows, and points out that the num¬ 
ber of these rows is variable, the number of lenses in the visual surface of 
each eye is variable, the number of lenses in successive rows is variable, 
a definite relation exists between the number of lenses of the eyes and the 
size (i. e. age) of the animal, the number of lenses increases from youth to 
maturity and decreases from maturity to senility. 
The unexpected fact of decrease in the number of lenses in old age is 
explained “ either by the gradual envelopment of the lenses of the upper 
margin by the sclera and palpebrum, and their entire concealment in the 
substance of the latter, unless it is possible that atrophy of the ommatidial 
nerve branches and concomitant reabsorption takes place with advanc¬ 
ing old age.” 
We quote the author’s conclusion: 
“ The study of the eye of Phacops rana, as here presented, allows the 
statement of the following points: 
1. The schizocroal eyes (such as occur in Phacops and its allies) of the 
trilobites are aggregated and not compound eyes. The visual organs of 
Harpes may prove to be of similar character. 
2. The scleral portion of the visual surface is of the same structure as 
the test, and is a direct continuation of it. 
3. No evidence appears of any continuous corneal layer covering the 
entire surface. 
4. The corneal lenses are wholly discrete from the epidermis, but are 
of epidermal origin. In the addition of new lenses to the visual surface, 
they appear to arise from a thinning of both surfaces of the integument. 
5. The corneal lenses were hollow or filled with some matter not hom¬ 
ogeneous with the cornea itself. 
6. The corneal lenses, and, therefore, the ommatidia, are added to the 
visual surface with advancing age until the mature growth of the indi¬ 
vidual is attained; thereafter they diminish in number, with increasing 
senility. 
7. The addition of corneal lenses occurs regularly at the extremities of 
the diagonal rows. 
8. No evidence is preserved of crystalline cones in the ommatidial 
cavities, though they may have been removed in the decomposition of the 
soft parts of the eye-” 
Geological Survey of the State of New York , Paleontology. Vol. vii. By 
James Hall, assisted by John M. Clarke. This volume worthily main¬ 
tains the reputation of the splendid series of which it forms a part. It is de¬ 
voted to the description and illustration of the trilobites and other Crustacea 
of the Upper Helderberg, Hamilton, Portage, Chemung and Catskill groups. 
There are 222 pages of descriptive text, preceded by lxiv pages devoted 
chiefly to a synopsis of genera. A carefully compiled synonymy of each 
genus is also a valuable feature. There are 127 Devonian species distrib¬ 
uted among 28 genera described in the volume, and in addition we have 
given descriptions of 17 species of Crustacea not Devonian. 
Geological Survey of the State of New York , Paleontology. Yol. v., Part 
ii. Supplement. By James Hall, This supplement is bound in with 
