ANNIVERSARY ADDRESS TO THE GEOLOGICAL SOCIETY 527 
Echinoids. But here it might be argued, on the other hand, that the 
spheroidal Echinoids, in reality, depart further from the general 
plan and from the embryonic form than the elongated Spatangoids 
do; and that the peculiar dental apparatus and the pedicellaria 
of the former are marks of at least as great differentiation as the 
petaloid ambulacra and semitz of the latter. 
Once more, the prevalence of Macrurous before Brachyurous 
Podophthalmia is apparently a fair piece of evidence in favour of 
progressive modification in the same order of Crustacea; and yet the 
case will not stand much sifting, seeing that the Macrurous Podoph- 
thalmia depart as far in one direction from the common type of 
Podophthalmia, or from any embryonic condition of the Brachyura, 
as the Brachyura do in the other; and that the middle terms be- 
tween Macrura and Brachyura—the Anomura—are little better re- 
presented in the older Mesozoic rocks than the Brachyura are. 
None of the cases of progressive modification which are cited from 
among the Invertebrata appear to me to have a foundation less open 
to criticism than these ; and if this be so, no careful reasoner would, 
I think, be inclined to lay a very great stress upon them. Among 
the Vertebrata, however, there are a few examples which appear to 
be far less open to objection. 
It is, in fact, true of several groups of Vertebrata which have lived 
through a considerable range of time, that the endoskeleton (more 
particularly the spinal column of the older genera) presents a less 
ossified, and so far less differentiated, condition than that of the 
younger genera. Thus the Devonian Ganoids, though almost all 
members of the same suborder as Polypterus, and presenting numer- 
ous important resemblances to the existing genus, which possesses 
biconcave vertebre, are, for the most part, wholly devoid of ossified 
vertebral centra. The Mesozoic Lepidosteide, again, have at most 
‘biconcave vertebrz, while the existing Lepzdosteus has Salamandroid, 
opisthoccelous, vertebrzee. So, none of the Paleozoic Sharks have 
shown themselves to be possessed of ossified vertebra, while the 
majority of modern Sharks possess such vertebra. Again, the more 
ancient Crocodilia and Lacertilia have vertebrae with the articular 
facets of their centra flattened or biconcave, while the modern mem- 
bers of the same group have them proccelous. But the most remark- 
able examples of progressive modification of the vertebral column, in 
correspondence with geological age, are those afforded by the 
Pycnodonts among fish, and the Labyrinthodonts among Amphibia. 
The late able ichthyologist Heckel pointed out the fact, that, while 
the Pycnodonts never possess true vertebral centra, they differ in the 
