348 PROCEEDINGS OF SECTION C. 
Trilobites form by far the most important part of the fossil 
fauna, and can generally be distinguished by the shortness of the 
pygidium. Some were as much as 18 inches in length. The 
minute Protocaris of the Lower Cambrian is a Phyllopod with a 
sub-quadrate carapace. 
The Cambrian mollusca are remarkable for the proportionately 
large number of elongated shells formerly classed as Pteropoda. 
Lately; however, many biologists have been led to the conclu- 
sion that the Pteropoda have had a comparatively late origin, 
and it is certainly remarkable that none are known between the 
Devonian and Eocene periods. It is now generally allowed that 
the Hyolithide, which includes all the pre-Cambrian and Cambrian 
forms, do not belong to the Pteropoda. They must, however, be 
considered as pelagic mollusca, and probably as the ancestors of 
the Cephalopoda. ‘True Cephalopoda appeared in the upper Cam- 
brian but did not attain any importance. Undoubted Gastropoda 
are represented by conical and spiral shells, all of which are very 
thin, and probably belonged to pelagic animals. The spiral shell 
is strongly in favour of these early Gastropods having been proso- 
branchiate, and this agrees with the discovery that some of the 
Opisthobranchs still inherit the twist in the visceral nerve-loop 
which is characteristic of the Prosobranchs. The early Peleecypoda 
were very minute, none of them being more than a quarter of an 
inch in length. The shells of all the Mollusca consist chiefly of a 
horny substance containing but a small quantity of phosphate of 
lime, and much less of carbonate of lime ; thus differing from the 
later shells which are composed almost entirely of calcic carbonate. 
No trace of a plant has been found in any of the Cambrian 
rocks unless Oldhamia and the Lower Cambrian oolitic limestones 
of South Australia* are proofs of the existence of calcareous alge. 
Eophyton, which was formerly thought to be a plant, has been 
shown to be due to the trailing of the oral lobes of a Medusa over 
soft mud. 
SPECULATIONS ON PRE-ORDOVICIAN LIFE. 
' Let us now see what these dry facts teach us. In the first place 
it is very remarkable that the only extensive pre-Cambrian fauna 
is composed of Radiolarians and Sponges ; and as the Sponges are 
more complex animals than the Radiolarians we must suppose 
that they are descendent from them and not the Radiolarians 
from the Sponges ; consequently the Radiolarians are the oldest 
organisms we know. No doubt the Radiolarians of Brittany 
are not the first of their class ; nevertheless we seem to have got 
as near, perhaps, as we ever can get to the first organisms, and 
we find that they belong to a group which at the present day 
* Pro. Linn., Soc. of N. 8. Wales, vol. xxi, pp. 671 and 574 (1897). 
