TRANSACTIONS OF SECTION D. 19 
the interval between the Carboniferous Period and the present day is relatively 
so small that it would require to be multiplied many times before we could 
expect the lines to meet in the common point, the ancestor of insects, to 
say nothing of the far more distant past, in which the Tracheate Arthropods 
met in an ancestor presenting many resemblances to Peripatus. But it must not 
be forgotten that all this vast undefined period is required for the history of one of 
the two grades of one of the three branches of the whole Phylum. 
Turning now to the brief consideration of the second grade of Arthropods, 
distinguished from the first grade by the absence of antenne, the Trilobites are 
probably the nearest approach to an ancestral form met with in the fossil state. 
Now that the possession of true antennee is certain, it is reasonable to suppose that 
the Trilobites represent an early Class of the Aceratous branch which had not yet 
become Aceratous. They are thus of the deepest interest in helping us to under- 
stand the origin of the antennaless branch, not by the ancestral absence, but by the 
loss of true antennz which formerly existed in the group. But the Trilobites did 
not themselves originate the other Classes, at any rate during Paleozoic times. 
They represent a large and dominant Class, presenting more of the characters of the 
common ancestor than the other Classes; but the latter had diverged and had 
become distinct long before the earliest fossiliferous rocks; for we find well-marked 
representatives of the Crustacea in Cambrian, and of the Arachnida in Silurian 
strata. The Trilobites, moreover, appear in the Cambrian with many distinct and 
very different forms, contained in upwards of forty genera, so that we are clearly 
very far from the origin of the group. 
Of the lower group of Crustacea, the Entomostraca, the Cirripedes are repre- 
sented by two genera in the Silurian, the Ostracodes by four genera in the Cambrian 
and over twenty in the Silurian; of these latter, two genera (Cythere and Bairdia) 
continue right through the fossiliferous series and exist at the present day. 
Remains of Phyllopods are more scanty, but can be traced in the Devonian and 
Carboniferous rocks. The early appearance of the Cirripedes is of especial interest, 
inasmuch as the fixed condition of these forms in the mature state is certainly not 
primitive, and yet, nevertheless, appears in the earliest representatives. 
The higher group, the Malacostraca, are represented by many genera of Phyl- 
locarida in the Silurian and Devonian, and two in the Cambrian. These also 
afford a good example of the imperfection of the record, inasmuch as no traces of 
the group are to be found between the Carboniferous and our existing fauna in 
which it is represented by the genus Nebalia. The Phyllocarida are recognised as 
the ancestors of the higher Malacostraca, and yet these latter already existed— 
in small numbers, it is true—side by side with the Phyllocarida in the Devonian. 
The evolution of the one into the other must have been much earlier. Here, as in 
the Arthropoda, we have evidence of progressive evolution among the highest 
groups of the Class, as we see in the comparatively late development of the Brachyura 
as compared with the Macrura. We find no trace of the origin of the Class, or of 
the larger groups into which it is divided, or, indeed, of the older among the small 
groupings into families and genera. 
Of the Arachnida, although some of the most wonderful examples of persistent 
types are to be found in this class, but little can be said. Merely to state the 
bare fact that three kinds of scorpion are found in the Silurian, two Pedipalpi, 
eight scorpions, and two spiders in the Carboniferous, is sufficient to show that the 
period computed by geologists must be immensely extended to account for the 
development of this Class alone, inasmuch as it existed in a highly specialised 
condition almost at the beginning of the fossiliferous series; while, as regards 
so extraordinarily complex an animal as a scorpion, nothing apparent in the way of 
progressive development has happened since. Professor Lankester has, however, 
pointed out to me that the Silurian scorpion Paleeophonus possessed heavier limbs 
than those of existing species, and this is a point in favour of an aquatic life like 
that of its near relation, Limulus. If so, it is probable that it possessed external 
1 For an account of the evolution of the Crustacea see the Presidential Addresses 
to the Geological Society in 1895 and 1896 by Dr. Henry Woodward. 
