Vol. II, Pt. I] VAN DENBURGH— GALAPAGOS TORTOISES 



293 



region varies much in shape. It may be narrow or broad, and 

 the pterygoid edges may be sharp or blunt. The alveolar 

 ridges also vary in position and degree of development. We 

 may safely say that no constant differences exist among the 

 skulls of the various races of Galapagoan tortoises. 



Key to Galapagoan Races 



a. — Saddle-backed races. — The sum of the percentage straight width, 

 curved width, width between second and third marginals, and the 

 difference between front and middle heights is less than, or exceeds 

 by not more than 10, the sum of the percentage straight length, front 

 height, twice the height to marginals, and length of plastron; or 

 else straight width less than 66%. 



b. — Plastron more than 72% ; percentage of plastron exceeds that of 

 straight width by not less than 3 ; distance between prominent 

 points of first marginals less than 30% ; front height usually less 

 than 53%?. 



c. — Plastron more than 88%o. Height to marginals 10 to 12%. 

 Hood Island. 



T. hoodensis. — p. 313. 

 C-. — Plastron not more than 88% ; 



d. — Width at 2nd to 3rd marginals less than 39%; eighth 

 marginal much reduced and wedgeshaped at top; height 

 to marginals 8 to 10%. Abingdon Island. 



T. abingdoni. — p. 296. 

 d^. — Width at 2nd to 3rd marginals not less than 39% ; 

 eighth marginal with a considerable superior margin. 

 e. — Width at 2nd to 3rd marginals plus middle height 

 ininus front height equals less than 59% ; 

 f. — Height to marginals greater, 7 to 12%; ; size small ; 

 plastron often shorter. Duncan Island. 



T. ephippium. — p. 306. 



f-. — Height to marginals less, 5 to 9% ; size large ; 

 plastron often longer. Northern Albemarle. 

 T. becki. — p. 303. 



e". — Width at 2nd to 3rd marginals plus middle height 

 minus front height equals not less than 60% (one 

 small 5 J. James Island. 



T. darwini. — p. 319. 



b^. — Plastron less than 72% ; percentage of plastron not exceeding that 

 of straight width ; distance between prominent points of first 

 marginals more than 30% ; front height more than 53% ; curved 

 length more than 123%. 

 Height to marginals less than 7%. Narborough Island. 



T. phantastica. — p. 299. 



a-. — Non-saddle-backed races. The sum of the percentage straight width, 

 curved width, width between second and third marginals and the 

 difference between front and middle heights exceeds by more than ten 

 the sum of the percentage straight length, front height, twice the 

 height to marginals, and plastron. 



bb. — The sum of the curved length, front height, middle height, and 

 plastron, equals or exceeds the sum of the straight length, straight 

 width and curved width; the front height is more than 41% of 

 the straight length. 



