LES ENCHA1NEMENTS DTJ MONDE ANIMAL 99 



shining enamel, and called, for this reason, ganoids, 

 we say that we are at the end of Primary or 

 at the commencement of Secondary times. The 

 assemblage in the same stratum of hard, bony-scaled 

 ganoids, ganoids with less bony scales, and finally, 

 fishes with soft scales, characterizes the middle 

 of Secondary times. Lastly, if there only remain 

 fishes with soft scales like our existing fishes, we 

 have the right to conclude that this stratum is at 

 the end of the Secondary or of a more modern age. 



The stages of evolution of the Reptiles afford us 

 analogous indications. In the Primary epoch the 

 ossification of their skeleton is very incomplete ; the 

 great bones of the limbs have not their extremities 

 well defined, and a thick cartilage represents the 

 heads of these bones ; in the same way, the centre 

 of the vertebrae is formed of distinct segments sur- 

 rounding a part of the dorsal cord which has re- 

 mained in the state of embryonic tissue ; they 

 are the unfinished quadrupeds. A little later, 

 types of reptiles somewhat akin to the preceding, 

 but larger, have an entirely ossified skeleton ; we 

 are then in the Trias. 



But it is doubtless in the Mammals that^these 

 stages of development have been most precisely 

 studied. . . . The Marsupial stage, that is, 

 the one in which the developments of the embryo 

 is partly extra-uterine, is common to the majority 

 of the Mammals of the Secondary era in Europe 

 and America. At the commencement of the Ter- 

 tiary epochs, the placental stage with complete 

 intra-uterine development begins to appear ; but 

 it is mixed up with the true Marsupials and with 



