TRANSACTIONS OF THE SECTIONS. 59 
each time of a ring, until the thorax has thus acquired seventeen free segments, and 
the pygidium two anchylosed segments, in all nineteen, which constitute the adult 
age. During the course of this evolution the form of the different parts of the body 
is developed in so continuous a manner, that in tracing the successive stages there 
is no sort of ‘ hiatus.” Towards the sixth stage four isolated grains are observed 
on each side of the glabella. I name ‘principal grain’ that which is nearest to the 
axis, and ‘ primitive grains’ the three other and smaller grains, which are arranged 
in a convex band towards the interior. Now, these four grains are persistent in 
all the following stages, both in their relative size and reciprocal position, with a 
constancy and regularity which alone might suffice to establish the specific identity 
of all these forms. The principal grain is also recognizable upon the adult, but at 
that age the three primitive grains become merged or lost amidst a crowd of other 
grains which accumulate around them. They all terminate in assuming a conical 
form, i. e, they become spines. [The figures show the details. | 
“Three other species have offered to me an analogous development, but with fewer 
intermediary stages between the extremes. These are the Zrinucleus ornatus (Stern- 
berg), Arionius ceticephalus (Barr.), and Arethusina Konincki (Barr.). i 
“The embryonic evolution out of the egg took place then in species or isolated 
genera among the trilobites of the Silurian epoch, just as among modern crustaceans. 
That which is remarkable is, that two of these four species belong to my lowest or 
primitive fauna, or to my band C, or the schists of Skrey, viz. Sao hirsuta and Ari- 
onius ceticephalus. You know that the Trinucleus ornatus characterizes my band D, 
or your Caradoc sandstone * ; and lastly, the drethusina Konincki is found exclusively 
at the base of my inferior limestone E, which occupies the place of your Wenlock 
formation. In the three other superior bands of my upper division of the Silurian 
system, no trilobite has offered to me a trace of a similar evolution. These all 
appear to be born with the complete number of thoracic segments, but not with all 
the articulations of the pygidium ¢. 
“‘T have thought that an acquaintance with this fact would be of some interest to 
you who first opened out the necropolis of trilobites. 
* M. Barrande does not consider, with Mr. Salter, that the Trinucleus Caractaci (Murch.) 
is the same species as the Trinucleus ornatus (Sternb.). 
tT The comments made by the eminent naturalist M. Milne-Edwards, Member of the Aca- 
demy of Sciencesin Paris, on this communication, must have so much weight, that a deviation 
is made from the ordinary practice in giving this abstract of them in a note.—* Prof. Milne- 
Edwards remarked that this discovery was equally interesting to the zoologist and physiolo- 
gist. Metamorphoses like those of the insect and tadpole were formerly supposed to be ex- 
ceptions to the ordinary rule, until the researches of Harvey showed, that the chick in the egg 
underwent changes quite as extensive and remarkable. It now appears to be a law of nature, 
that animals are more alike as they are observed at a period nearer their embryonic state ; 
and it is of the highest consideration in zoology to show, through what stages animals pass 
before arriving at their adult form. The zoological affinities of the trilobites were long a 
Matter of dispute. They were first supposed to be Chitons, until Alexander Brongniart 
showed they were true crustaceans. But the crustacean forms are very varied, and it has 
been held uncertain whether the trilobites were allied to certain Isopoda, se. Oniscus, or, as 
Mr. Thompson suggests, to Apus. Barrande’s observations confirm the views of Mr. Thomp- 
son. The Isopods are born almost with the same form which they retain through life; but 
the Apus quits the egg in an imperfect state, having but few of the segments which consti- 
tute the body of the adult. In the young Sao the number of thoracic segments continually 
increased until the animal was adult; and to each of these (though no traces are now seen) 
legs were certainly affixed, not like the hard legs of insects adapted to terrestrial movement, 
but soft and membranous like those of Apus, for swimming in the water. The cephalic 
shield, which in the youngest stage of Sao formed the whole animal, constitutes but a small 
portion of the adult; and the amount of change exhibited in successive stages of development 
is so great, that it would be no wonder if zoologists should have built up upon it numerous 
‘species and even several genera. The rule which obtains now, that animals belonging to the 
same zoological type, though much differing in the adult, lose those differences in states ap- 
proaching their embryonic condition, is seen even in the remains of animals which perished 
in the most remote epochs; and thus the tenants of the Silurian seas furnish arguments 

hitherto afforded by the study of living animals alone.” 
