392 PERSISTENT TYPES OF LIFE 



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 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, 

 opisthoccelous, vertebrae. So, none of the Paleozoic Sharks 

 have shown themselves to be possessed of ossified verte- 

 bra, while the majority of modern Sharks possess such 

 vertebrae. Again, the more ancient Crocodilia 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 remarkable 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 exhibit- 

 ing hardly any such expansion, while the Mesozoic genera 

 present a greater and greater development, 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 

 Archegosaurus had very imperfectly developed vertebral 

 centra, while the Triassic Mastodonsaurus had the same 

 parts completely ossified.* 



As the Address is passing through the press (March 7, 1862), 

 evidence lies before me of the existence of a new Labyrinthodont 

 (Pholidogasler), from the Edinburgh coal-fleld, with well-ossified 

 vertebral centra. 



