66 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 16, 
No doubt, I think, can exist that if the rocks of Yass Plains are 
not Silurian, they are at the very base of the Devonian system; and 
the occurrence of the Yarralumla Trilobites leads to the same conclu- 
sion. The existence of Trinuclei at Yarralumla, and especially of one 
species, which I cannot separate from T'.. Caractaci, is an important 
fact which must not be lost sight of. 
Although but few of the fossils at Burragood, Colocolo and Yar- 
ralumla are identified, yet I cannot but express my opinion that if 
facilities of comparison (possessed by geologists in Europe) were 
afforded to inquirers here, by the existence of cabinets, to the con- 
tents of which reference could be made, numerous other identifica- 
tions would probably be made out, and the existence of new species 
and genera be determined out of 1500 or 1600 species which I have 
collected from various deposits in New South Wales, many of which 
(owing to the brittle character of the matrix, in which they occur 
as mere casts) are perishable. 
On comparing together the preceding observations, it will be found 
that the preponderating evidence is in favour of the conclusion that 
the beds furnishing the Trilobites of the Paterson and the Murrum- 
bidgee are related more nearly to the Silurian than to the Devonian 
rocks of Europe. 
Notwithstanding my admiration of the intelligence and perseverance 
exhibited in the geological portion of the ‘ Physical Description,’ it 
appears to me that a great modification of the “‘ epochs” there as- 
sumed must take place before we shall thoroughly understand the 
true history of the Australian system. Should any actual division 
into “ Epochs’’ be determined, it appears to me that such division 
would more accurately be fixed by a separation of the Burragood, 
Colocolo and Yarralumla beds from those of Wollongong, Raymond 
Terrace, Harpur’s Hill and Mulberrmg Creek. Yet, as before 
mentioned, all these betray an imsensible gradation into the beds 
charged with Orthides, Atrypze and Trilobites ; the Wollongong beds, 
in the cliffs of Munniwarree, passmg in the southern part of Illa- 
warra, as on the coast near Wallamboola and Wagamee, into others 
charged with fossils more nearly allied to those of Yarralumla and 
- Burragood. | 
9. On TREMATIS, a new genus belonging to the family of Bracuto- 
popous Motiusca. By Danie SHarpe, Esq., F.G 
In looking over the valuable collection of Paleeozoic fossils brought 
from the United States by Mr. Lyell, I was struck by a small Bra- 
chiopod possessing a combination of structure hitherto unobserved, 
which entitles it to be placed in a separate genus. 
The general appearance of the shell is similar to Orbccu/a, in which 
genus it has hitherto been placed. It is nearly circular, but slightly 
broader than long and with an irregular outline. The shell is attached 
by a ligament which passes through an oblong slit in the lower valve, 
