36 GEOLOGICAL MEMOIRS. 
many new species have been discovered, some in Sweden and England, 
others in North America, and others even in the vicmity of the Cape 
of Good Hope, but the mdividuals in each locality were always far 
from being abundant. The researches of M. Rouault, however, show 
that in certain poimts at least Trilobites were formerly as common as 
the crabs on our coasts are at present; forin examining, durmg a few 
weeks only, the environs of Polign¢, that skilful collector has suc- 
ceeded in obtaining more than two thousand specimens of one species, 
Trinucleus Pongerardit. 
The fossils found so abundantly in these localities are not all in the 
same state of preservation. In some the shell is wholly transformed 
into sulphuret of iron ; m others a portion only of the tegumentary _ 
skeleton has undergone a change of this nature ; whilst in others 
again sulphuret of iron forms no part whatever of the solid envelope. 
M. Rouault has sought for the reason of these differences, and on 
comparing the structure of recent shells with that of the remaims of 
molluses thus modified, he finds that the species, in which the process 
of fossilization has been accompanied with a molecular deposition of 
sulphuret of iron in the substance of the tissues, are those into whose 
composition a large amount of carbonate of lime entered; whilst 
those whose original texture was horny have not undergone a similar 
change. Then, applying these data to the study of Trilobites, he has 
endeavoured to determine the original structure of the external ske- 
leton of these animals from the nature of the transformations it has 
undergone in the interior of the earth. 
Calymene and Phacops always appear with a shell formed of sul- 
phuret of iron; Nileus, Illenus, Ogygia, Cheirurus and Prionocheilus 
never offer any trace of a similar transformation. Lastly, m Trinu- 
cleus M. Rouault has constantly found certain parts converted ito 
sulphuret of iron, whilst other parts have suffered no analogous 
modification. Hence he concludes that in Calymene and Phacops 
the shell was calcareous, like the carapace of our crabs and lobsters ; 
whilst in Nileus, Hlzenus, Ogygia, &c., the tegumentary skeleton was 
membranous or horny ; and im the Trinucleus the greater portion of 
the body had a structure analogous to that of the existing Apus or 
Branchipus, whilst the spimiform prolongations of the cephalic buckler 
were calcareous. The author also considers these differences as cor- 
responding to the natural divisions in the great family of the Trilobites; 
and he adds to his memoir a table, in which he classes these fossils 
from the characters we have now mentioned. We do not wholly par- 
ticipate in the opmion of M. Rouault on this pomt; but we agree 
with him that the indications furnished by the presence or the 
absence of sulphuret of iron in the shells of fossil animals deserve the 
close attention of zoologists, and may aid us in determining the kind 
of structure peculiar to different species. 
M. Rouault’s observations also throw some new light on the ana- 
tomical constitution of the eyes of Trilobites. It is well known that 
in Calymene [ Phacops?] and several other genera of the same family, 
there exists, on each side in front, a large reticulated or compound 
eye. The specimens collected by M. Rouault show that these com- 
