352 PROCEEDINGS OF SECTION C. 
during the Ordovician period decreased in number, and became 
regularly arranged ; while throughout the greater part of the 
Silurian we find only simple, unbranched forms. The thecz also, 
which were at first straight and with straight apertures, became 
curved and with curved apertures often produced into a spine, 
and in the Silurian period the aperture was in some cases still 
more complex. The species of Graptolites are widely spread 
horizontally, and occur in very dissimilar rocks, such as limestones, 
shales, and grits. Sometimes they are accompanied by a varied 
fauna ; but in other places they occur in thin zones without any 
other fossils, while the different species which characterise these 
zones are the same, and have the same vertical distribution, 
wherever they are found. The explanation of these facts appears 
to be that the Graptolites floated on the surface, and consequently 
were independent of the depth of the sea and the nature of the 
sea bottom. We find additional evidence in confirmation of this 
in the fact that some of the early species were furnished with a 
disc, which probably acted as a float. 
The Brachiopoda show a remarkable development during both 
the periods under consideration. Orthis, in which the triangular 
opening for the peduncle remains open all through life, gave rise 
to the Rhynchonellide, which has a pair of deltidial plates in the 
opening, and to the Strophomenide, in which the opening becomes, 
during growth, entirely closed by a shelly plate, thus leaving the 
animal free. From the Rhynchonellide sprung, in the Silurian 
period, the Terebratulide, in which the deltidial plates remain 
separate, and the Spiriferide, in which they unite during growth, 
and close the opening for the peduncle, as in the Strophomenide. 
All classes of the Mollusca increased greatly, especially the 
Cephalopoda, which are, after the Brachiopods and Trilobites, the 
most numerous of Silurian fossils. Now, too, we find, in the Lower 
Ordovician, thick-shelled Gastropods, and in the Silurian we have 
Chiton, an ancient form of soft-bodied mollusc, specially modified 
for protection among the waves of the shore, It has been sug- 
gested on very good authority—S. P. Woodward, H. von Jhering, 
and Professor A. Hyatt—that Tentaculites, and, perhaps, Hyolathes, 
represent the primitive Cephalopoda. Anyhow, it is highly pro- 
bable that the first Cephalopods were pelagic in habit, for we 
know no ground-animals from which they could have been derived. 
These pelagic Cephalopods are but little known, and possibly some 
of the conodonts belonged to them. -The ground Cephalopods 
appear first as Nautiloidea, which were very rare in the Upper 
Cambrian, increased rapidly in numbers during the Ordovician, 
and attained their maximum development in the Silurian. 
The Arthropoda were now reinforced by the Eurypterida, in 
the Ordovician, and the Xiphosura in the Upper Salurian. The 
former were the largest and most powerful animals of their day, 
