ROUAULT ON TRILOBITES. 37 
pound eyes were provided with a transparent reticulated cornea, 
-with a layer of lenses ; and deeper still, with a layer of reversed cones 
with concave bases serving as supports for these kinds of crystalline 
lenses. M. Rouault directs the attention of zoologists to the differ- 
ences that exist in the forms of the cornea, of the lenses, or of the 
cones, according to the species, and insists with much reason on that 
character being employed in the specific determination of Trilobites. 
The author has also made known the manner in which the Trinu- 
cleus rolled itself up, which was altogether different from what is ob- 
served in the Calymene or the Nileus. Lastly, he adds several curi- 
ous details relative to the anatomical history of these animals. 
The fossils collected by M. Rouault also enable the geologist to de- 
termine precisely the age of the formation from which he has pro- 
cured them; and on this point confirm the results already obtained 
by MM. de Verneuil and d’Archiac. We may thence infer that the 
slates of Angers, Hunaudiére, Bain, Poligné and Vitré, are contempo- 
raneous, and belong to the lower Silurian system, whilst the lime- 
stones and the slates of Gahard, near Rennes, appear to be of the same 
age with the Devonian formations of the Eifel. 
This short exposition shows that the researches of M. Rouault 
should interest us in more than one point of view; and we are per- 
suaded, from the results he has already obtained, that if this young 
naturalist is enabled to pursue those geological studies to which he 
has devoted himself with so much zeal, he cannot fail to render new 
services to paleeontology. [J. N.] 
To the above report of Professor Milne-Edwards we add a few 
extracts from the original memoir now published in the Bulletin of 
the Geological Society of France, tom. iv. p. 309. 
Trinucleus Pongerardi, 1. c. pl. ui. fig. 1.—Buckler truly semi- 
circular, nearly straight behind; glabella and cheeks smooth, the 
former largest ; fringe [bourrelet] hollow, formed of a double mem- 
brane, more separated in the centre than the edges, like a double 
convex lens, and pierced by six rows of pores, of which two rows are 
absent in front, the sides of the pores forming hollow pillars to keep 
the surfaces apart (probably a floatmg organ). Head spines longer 
than the entire body, slightly bent towards the end, quadrangular, 
the outer angle contmued from the outer edge of the fringe, the mner 
from the corresponding edge, and two others from a ridge between 
the first and second rows of pores; the spines apparently hollow all 
along, bifurcate in two out of five specimens of all ages, and either on 
the right or left spine, or both ; on any part of the spine, but gene- 
rally at two-thirds its length; the bifurcation of the spine varies, 
sometimes the two ends are equally divaricate, sometimes there is an 
outer, sometimes an inner branch. 
Abdomen and post-abdomen so much smaller than the head, that 
although a five-franc piece would scarcely cover the head, a five-cen- 
times would more than cover body and tail. Axal lobe of the abdo- 
men equal to the side lobes; six furrowed segments ; post-abdomen 
four times as broad as long, axis not very strongly marked, tolerably 
uo 
