TRANSACTIONS OF SECTION D. 827 
gills, not yet inverted to form the lung-book. The Merostomata are of course a 
Paleozoic group, and reach their highest: known development at their first appear- 
ance in the Silurian ; since then they have done nothing but disappear gradually, 
leaying the single genus Limulus, unmodified since its first appearance in the 
Trias, to represent them. It is impossible to find clearer evidence of the decline 
rather than the rise of a group. No progressive development, but a gradual cr 
rapid extinction, and consequent reduction in the number of genera and species, is 
a summary of the record of the fossiliferous rocks as regards this group and many 
others, such as the Trilobites, the Brachiopods, and the Nautilide. All these 
groups begin with many forms in the oldest fossiliferous rocks, and three of them 
have left genera practically unchanged from their first appearance to the present 
day. What must have been the time required to carry through the vast amount 
of structural change implied in the origin of these persistent types and the groups 
to which they belong—a period so extended that the interval between the oldest 
Paleozoic rocks and the present day supplies no measurable unit ! 
But I am digressing from the Appendiculate Phylum. We have seen that the 
fossil record is unusually complete as regards two classes in each grade of the 
Arthropod branch, but that these classes were well developed and flourishing in 
Paleozoic times. The only evidence of progressive evolution is in the development 
of the highest orders and families of the classes. Of the origin of the classes 
nothing is told, and we can hardly escape the conclusion that for the development 
of the Arthropod branches from a common Chetopod-like ancestor, and for the 
further development of the classes of each branch, a period many times the length 
of the fossiliferous series is required, judging from the insignificant amount of 
development which has taken place during the formation of this series. 
It is impossible to consider the other Coelomate Phyla as I have done the 
Appendiculata. I can only briefly state the conclusions to which we are led. 
As regards the Molluscan Phylum, the evidence is perhaps even stronger than 
in the Appendiculata. Representatives of the whole of the classes are, it is believed, 
found in the Cambrian or Lower Silurian. The Pteropods are generally admitted 
to be a recent modification of the Gastropods, and yet, if the fossils described in the 
genera Conularia, Hyolithes, Pterotheca, &c., are true Pteropods, as they are 
supposed to be, they occur in the Cambrian and Silurian strata, while the group 
of Gastropods from which they almost certainly arose, the Bullide, are not known 
before the Trias. Furthermore, the forms which are clearly the oldest of the 
Pteropods—Limacina and Spirialis—are not known before the beginning of the 
Tertiary period. Either there is a mistake in the identification of the Paleozoic 
fossils as Pteropods, or the record is even more incomplete than usual, and the 
most specialised of all Molluscan groups had been formed before the date of the 
-earliest fossiliferous rocks. Even if this should hereafter be disproved, there can be 
no doubt about the early appearance of the Molluscan classes, and that it is the 
irony of an incomplete record which places the Cephalopods and Gastropods in the 
Cambrian, and the far more ancestral Chiton no lower than the Silurian. Through- 
out the fossiliferous series the older families of Gastropods and Lamellibranchs are 
followed by numerous other families, which were doubtless derived from them ; 
new and higher groups of Cephalopods were developed, and, with the older groups, 
either persisted until the present time or became extinct. But in all this splitting 
up of the classes into groups of not widely different morphological value, there is 
very little’ progressive modification ; and, taking such changes in such a period as 
our unit for the determination of the time which was necessary for the origin of 
the classes from a form like Chiton, we are led to the same conclusion as 
that which followed from the consideration of the Appendiculata, viz., that 
the ego series would have to be multiplied several times in order to 
rovide it, 
; Of the Phylum Gephyrea I will only mention the Brachiopods, which are 
found in immense profusion in the early Paleozoic rocks and which have occupied 
the subsequent time in becoming less dominant and important. So far from 
helping us to clear up the mystery which surrounds the origin of the class, the 
earliest forms are quite as specialised as those living now, and, some of them (Lingula, 
3H 2 
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