A.NN1VEKSAET ADDEESS. liu 



the Yertebrata, however, there are a few examples which appear to 

 be far less open to objection. 



It is, in fact, true of several groups of Yertebrata 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 difterentiated, condition than that of the 

 younger genera. Thus the Devonian Ganoids, though almost all 

 members of the same suborder as Fohjpterus, and presenting nume- 

 rous important resemblances to the existing genus, which possesses 

 biconcave vertebrae, are, for the most part, wholly devoid of ossified 

 vertebral centra. The ^lesozoic Lepidosteidse, again, have at most 

 biconcave vertebree, 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 vertebrae with the articular 

 facets of their centra flattened or biconcave, while the modem mem- 

 bers of the same group have them procoelous. Eut the most remark- 

 able examples of progressive modification of the vertebral column, 

 in correspondence with geological age, are those afi^orded 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 difier in the 

 degree of expansion and extension of the ends of the bony arches of 

 the vertebras upon the sheath of the notochord ; the Carboniferous 

 forms exliibiting 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 know- 

 ledge of the organization of the older Labyrinthodonts, has proved 

 that the Carboniferous Arclicgosanrus had very imperfectly deve- 

 loped vertebral centra, while the Triassic Mastodonsaurus had the 

 same parts completely ossified*. 



The regularity and evenness of the dentition of the Anoploihe" 

 rium as contrasted with that of existing Artiodactyles, and the 

 assumed nearer approach of the dentition of certain ancient Carni- 

 vores to the typical arrangement, have also been cited as exempli- 

 fications of a law of progressive development, but I know of no 

 other cases based on positive evidence which arc worthy of particular 

 notice. 



^Vhat then does an impartial survey of the positively ascertained 

 truths of palaeontology testify in relation to the common doctrines 

 of progressive modification, which suppose that modification to have 

 taken place by a necessar}- progress from more to less embryonic 

 forms, or from more to less generalized types, within the limits of 

 the peiiod represented by the fossiliferous rocks ? 



* A8 this Address is passing through the press (March 7, 18G2), evidence lies 

 before me of the existence of a new Labyrinthodont {Pholidof/oMer), from the 

 Edinburgh coal-field, with well-ossified Tcrtebral centra. 



