ADAPTATIONS TO SPECIAL ENDS 



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sphenes and the cavernous zygantra, the former of which fit 

 into the latter after the fashion of a tenon-and-mortice joint. 

 Neither is it a matter for wonder to find similar articular sur- 

 faces developed in the vertebrae of the extinct sea-serpents, or Py- 

 thonomorpha, as well as in some of the members of the group 

 Dolichosauria, which is likewise extinct. What does give rise 



Fig. io. — Ventral view of the skeleton of the existing Logger-head Turtle 

 (Tlialassochelys) with the plastron removed, to show the position of the shoulder- 

 girdle and pelvis within the ribs. From Guide to Fossil Reptiles, Amphibians, 

 and Fishes in British Museum (Natural History). 



to wonderment, is, however, the fact that while these zygosphenes 

 are absent in the Anguidce (slow- worm and glass-snake), except 

 in a rudimentary condition in one genus, they are fully developed 

 in the iguanas. If snakes require them, why are they also not 

 essential to glass-snakes and blind-worms, and if monitors and 

 other lizards can dispense with them, why are they developed 

 in iguanas ? Possibly the answer to the first part of the question 

 is that the Anguidce do not climb, and that additional articu- 



