58 REPORT—1849. 
Sirponotretez, Kutorga. 
A. With a tubular closed sipho. : 
a, The external siphonal opening passes from the apex towards the anterior 
margin. 
1. Siphonotreta, De Verneuil. 
b. The siphonal opening is directed from the apex towards the dorsal margin. 
2. Schizotreta, Kutorga (Orbiculotdea, D’Orb.). 
Opening narrow, slit-like ; no area, nor mark of deltidium. 
8. Acrotreta, Kutorga. 
Opening elongated oval; area triangular and flattened, with a deltidium-like 
furrow, 
B. With a furrow-like sipho, opened on the whole hinge plain. 
4. Aulonotreta, Kutorga (Obolus, Kichw.; Ungula, Pander). 
The author adds a series of critical remarks on the above groups, noticing some 
peculiarities of their geographical and geological position, and concludes by charac- 
terizing the new species of Siphonotreta. 
SIPHONOTRETA? ANGLICA. 
Shell of a rather oval form, depressed, marked by fine lines of growth; surface mi- 
nutely but concentrically reticulated, reticulation regular with quadrangular areole, 
and covered with many slender linear tubular spines or their bases, somewhat quin- 
cuncially arranged ; spines smooth, dilated at the base, a little above which they re- 
main of nearly uniform size throughout, and are regularly and transversely suleated 
or contracted, giving the spines a beaded or jointed appearance. 
The general form of this shell and quincuncial arrangement of the spines resemble 
§. aculeata, Kutorga; but as that author does not figure or allude to any reticulated 
structure or the moniliform spines*, this is considered to be distinct; unfortunately the 
specimen is compressed, so that all the characters are not fully shown, 
On the Metamorphosis of certain Trilobites as recently discovered by M. Bar- 
rande. (Communicated by Sir Roperick Impry Murcaison.) 
Sir Roderick Murchison brought before the Section the important discovery made 
by M. Barrande, of the metamorphosis of Trilobites, as exhibited in a series of forms 
apparently very distinct, but which have been shown by that author to belong to the 
one species Sao hirsuta (Barr.). Referring in the first instance to the extraordinary 
number of species of Trilobites recently discovered in the paleozoic rocks of Bo- 
hemia, whether as compared with the small number hitherto known in that tract, 
or the whole quantity described in other parts of the world, Sir Roderick explained 
how with untiring zeal and ability, and at considerable cost and labour, M. Bar- 
rande had been the real agent in opening out this rich field, and how by a long and 
careful analysis of all its organic remains he had shown that it is essentially of Silu- 
rian age. In anticipation of a great work by M. Barrande (the ‘ Silurian Rocks of 
Bohemia’), in which the necessary proofs will be given, and containing among 
numerous other illustrations 40 plates of Trilobites only, Sir Roderick communicated 
the following extract of a letter from that author :— 
«The fact, which is made intelligible by the plate of drawings annexed‘, relates 
to a species which I have named Sao hirsutu, of which I have verified the gradual 
development from the embryonic to the adult state. I have been able to discover 
twenty successive stages in this progress, which took place after development from 
the egg, as is observed in some of our modern crustaceans. The first stage is marked 
by a disc two-thirds of a millimetre in diameter, of which the head only occupies 
the whole of the trilobed surface. In the second stage the thorax appears in a 
rudimentary state, and it increases in the following stages by the successive addition 
* The moniliform character of the spines may not be peculiar to this species, but will pro- 
bably be found to belong to the whole genus, when the spines are carefully examined by a 
higher power than that used by Dr. Kutorga. 
T Of these an enlarged diagram by Mr. Salter was exhibited. 

