24 



WATER REPTILES OF THE PAST AND PRESENT 



have the single opening high up on the side, corresponding 

 apparently to the supratemporal vacuity of the double-arched 

 forms, as those with two openings are called. Many others, how- 

 ever, like the whole order Therapsida and the Theromorpha, have 

 the single opening lower down and bounded differently; their 

 relationships are doubtful, since it is very much of a question how 

 the single opening has arisen. There have been many theories 

 to account for the origin of the temporal vacuities, but all are yet 

 speculations. Notwithstanding these doubts, which more recent 

 discoveries have intensified, there can be none that the structure 



Fig. 8. — Spkenodon (tuatera). Skull from side and above: pm, premaxiUa; 

 n, nasal; prf, prefrontal; /, frontal; pf, postfrontal; p, parietal; po, postorbital; 

 sq, squamosal; m, maxiUa; /, jugal; qj, quadratojugal; q, quadrate; c, coronoid; 

 sa, surangular; art, articular; pa, prearticular; d, dentary; an, angular. 



of this region of the skull offers important and reliable characters 

 for the classification of the reptiles into the larger groups, but, 

 unfortunately, we are very uncertain yet as to what this classi- 

 fication should be. We are confident that all those reptiles having 

 two temporal vacuities on each side, are related to each other; we 

 are yet very much in doubt as to the classification of all other 

 reptiles, or at least all others having only a single temporal vacuity 

 on each side. 



Better evidences of relationships, or the absence of relation- 

 ships, are offered by the presence of certain bones in the skulls 

 in some orders that are lost in others, since it may be accepted as 



