x .] PERSISTENT TYPES OF LIFE. 195 



less open to criticism than these ; and if this be so, no careful 

 reasoner would, I think, be inclined 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 biconclave vertebra, are, for the most part, 

 wholly devoid of ossified vertebral centra. The Mesozoic 

 Lepidosteidae, again, have, at most, biconcave vertebra, while the 

 existing Lepidosteus has Salamandroid, opisthoccelous, vertebrae. 

 So, none of the Palaeozoic Sharks have shown themselves to be 

 possessed of ossified vertebra, while the majority of modern 

 Sharks possess such vertebrae. Again, the more ancient Croco- 

 dilia and Lacertilia have vertebrae with the articular facets of 

 their centra flattened or biconcave, while the modern members 

 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 degree of expansion and extension of the ends of 

 the bony arches of the vertebrae upon the sheath of the notochord ; 

 the Carboniferous forms exhibiting hardly any such expansion, 

 while the Mesozoic genera present a greater and greater develop 

 ment, until, in the Tertiary forms, the expanded ends become 

 suturally united so as to form a sort of false vertebra. Hermann 

 von Meyer, again, to whose luminous researches we are indebted 

 for our present large knowledge of the organization of the older 

 Labyrinthodonts, has proved that the Carboniferous Arckegcsaurus 



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