In efts, as Professor Gegenbaur himself has pointed out,[181] there is a striking correspondence between the bones or cartilages supporting the arm, wrist, and fingers, and those sustaining the leg, ankle, and toes, with the exception that the toes exceed the fingers in number by one.
[Illustration: SKELETON OF AN ICHTHYOSAURUS.] [Illustration: A.SKELETON OF ANTERIOR EXTREMITY OF AN EFT.
B.SKELETON OF POSTERIOR EXTREMITY OF THE SAME.] Yet these animals are far from being the root-forms from which all the Vertebrata have diverged, as is evidenced from the degree of specialization which their structure presents.

If they have descended from such {178} primitive forms as Professor Gegenbaur imagines, then they have built up a secondary serial homology--a repetition of similar modifications--fully as remarkable as if it were primary.

The Plesiosauria--those extinct marine reptiles of the Secondary period, with long necks, small heads, and paddle-like limbs--are of yet higher organization than are the efts and other Amphibia.

Nevertheless they present us with a similarity of structure between the fore and hind limb, which is so great as almost to be {179} identity.

But the Amphibia and Plesiosauria, though not themselves primitive vertebrate types, may be thought by some to have derived their limb-structure by direct descent from such.