224 ^ag Sermons, ssags, m\b HUbbhu. [x. 



of the Brachyura, as the Brachyura do in the other ; 

 and that the middle terms between Macrura and 

 Brachyura the Anomura are little better represented 

 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 in- 

 clined to lay 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 sub-order as Polypterus, 

 and presenting numerous important resemblances to the 

 existing genus, which possesses biconcave vertebrae, are, 

 for the most part, wholly devoid of ossified vertebral 

 centra. The Mesozoic Lepidosteidae, again, have, at most, 

 biconcave vertebrae, while the existing Lepidosteus has 

 Salamandroid, opisthocoelous, vertebrae. So, none of the 

 Palaeozoic Sharks have shown themselves to be possessed 

 of ossified vertebrae, while the majority of modern 

 Sharks possess such vertebrae. Again, the more ancient 

 Crocodilia and Lacertilia have vertebras with the articular 

 facets of their centra flattened or biconcave, while 

 the modern members of the same group have them 

 proco3lous. But the most remarkable examples of 

 progressive modification of the vertebral column, in cor- 

 respondence with geological age, are those afforded by 

 the Pycnodonts among fish, and the Labyrinthodonts 

 among Amphibia. 



