NO. 1540. THE SKULL OF BRACHAUCHENLVS—WILLLSTON. 487 



loss of the caudal fin results in the total disability of the animal. In 

 animals propelling; themselves wholly or chiefly hj the aid of the 

 limbs the propodials are not shortened, but are, on the contrary, 

 elongated, as in the plesiosaurs and marine turtles. The reason is 

 obvious: The propodials become elongated handles of oar-like organs, 

 of which the blades are formed by the progressively widened epi-, 

 meso-, and metapodial elements, and the phalanges. The front limbs 

 of the plesiosaurs are always broader and stronger, but not longer 

 than the hind ones. The front legs of the marine turtles are not only 

 broader and stronger, but also longer than the liind ones, though the 

 latter have by no means lost their eflPectiveness as propelling, or, more 

 probably, guiding organs. The connection of the hind limbs of the 

 plesiosaurs with a well-developed sacrum of three vertebrae conclu- 

 sively proves the propelling function of these limbs, if such proof were 

 not abundantly furnished by the limbs themselves. 



We have, then, certain marked resemblances in the form and mode 

 of progression between the plesiosaurs and turtles, as contrasted with 

 the tail-propelling type presented by the ichthyosaurs, mosasaurs, 

 and thalattosuchians; and Fraas uses" tliis resemblance as a support 

 for the diphyletic grouping of the reptilia by Osborn into the Synap- 

 sida and Diapsida, the former having the oar-propelling type, the 

 latter the tail-propelling type. But the argument is fallacious; the 

 resemblances in mode of progression and bodily form no more imply 

 a common phyletic origin than do the much more marked resem- 

 blances of the ichthyosaurs and dolphins. 



It is chiefly because of the external resemblances of form and 

 similarity in mode of locomotion in the water that it has been gener- 

 ally and indefinitely assumed, from Buckland's time to the present 

 that the plesiosaurs were related to the turtles. How well this 

 hypothesis is sustafeied by the internal structure may be shown by 

 the following comments : 



In addition to external resemblances and undoubtetl similarity in 

 habits of life, two other characters have been often cited as evidence 

 of relationship between these two orders — the epiphysial mode of 

 ossification of the propodials (or rather of the humeri, since there is 

 no evidence yet that the femora have the peculiar ''epiphyses"), and 

 the fusion of the procoracoid with the scapula. As to the fii-st of 

 these assertions, recent careful investigations by R. Moodie con- 

 clusively prove that the turtles do not have true epiphyses, and as 

 was long ago stated by Dollo, and recently confii-med by Mr. Moodie, 

 the lizards do have, many of them, at least, distinct terminal bony 

 epiphyses on then- long bones. The mode of ossification of the 

 humerus of the plesiosaurs is most extraordinary, without known 

 parallel among reptiles, or mammals either, so far as that is con- 



« Jahreshef len d. Vereins f. vaterl. Naturkunde in Wurteinberg, 1905, p. 363. 



