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. Tortoises, however, must be admitted to be not only highly differentiated organisms, but to be far indeed removed from primeval vertebrate structure. Yet certain tortoises[182] (notably _Chelydra Temminckii_) exhibit such a remarkable uniformity in fore and hind limb structure (extending even up to the proximal ends of the humerus and femur) that it is impossible to doubt its independent development in these forms. [Illustration: SKELETON OF A PLESIOSAURUS.] Again in the Potto (Perodicticus) there is an extra bone in the foot, situated in the transverse ligament enclosing the flexor tendons. It is noteworthy that in the _hand_ of the same animal a serially homologous structure should also be developed.[183] In the allied form called the slow lemur (Nycticebus) we have certain arrangements of the muscles and tendons of the hand which reproduce in great measure those of the foot and _vice versa_.[184] And in the Hyrax another myological resemblance appears.[185] It is, however, needless to multiply instances which can easily be produced in large numbers if required. Secondly, with regard to teratology, it is notorious that similar abnormalities are often found to co-exist in both the pelvic and thoracic limbs. M. Isidore Geoffroy St. Hilaire remarks,[186] "L'anomalie se repete d'un membre thoracique au membre abdominal du meme cote." And he afterwards quotes from Weitbrecht,[187] who had "observe dans un cas l'absence simultanee aux deux mains et aux deux pieds, de quelques doigts, de {180} quelques metacarpiens et metatarsiens, enfin de quelques os du carpe et du tarse." [Illustration: LONG FLEXOR MUSCLES AND TENDONS OF THE HAND. _P.t._ Pronator teres. _F.s._ Flexor sublimis digitorum. _F.p._ Flexor profundus digitorum. _F.l.p._ Flexor longus pollicis.]