NO. 1540. THE SK ULL OF BRA CHA VCHENIVS— WILLISTOX. 489 



wanting in the turtles. The turtles have a single, unpaired, true 

 vomer and no pre vomers; the sauropterygians have large pre vomers 

 and a small or no true vomer. A large interpterygoid vacuit}^ is 

 present in the plesiosaurs, wanting in the turtles. Furthermore, the 

 turtles have still preserved the primitive hypocentral mode of attach- 

 ment of the thoracic ribs, while the single-headed thoracic ril^s of the 

 plesiosaurs are attached higli uj) on the extremities of the diapophy- 

 ses, and this character can not be ascribed to aquatic adaptation, I 

 think, since the ichthyosaiu's and mosasaiu's have preserved their 

 early pleurocentral attachment of these ribs. 



And one is welcome to all the resemblances that may be found in 

 the vertebrae, girdles, and limbs. I repeat, there is only a remote 

 relationship between the two orders in osteological structure. The 

 plesiosaurs coidd not have been derived from any ancestors that 

 might by the widest stretch of imagination be called Chelonia, or 

 Chelonia-like. Nor could the turtles have come from any forbears 

 even suggesting the sauropterygian structure. 



I am still strongly of the opinion that the Sauro])terygia were 

 derived from a primitive therocephalian ancestry; while I am fu-mly 

 of the opinion that the turtles have had a quite independent origin 

 from some primitive cotylosaiunan, like the Chelydosauria, as Case 

 has forcefully shown. The turtles occupy a phylum distinctly then 

 own, no more intimately related to the ])lesiosain's than they are to 

 the ichthyosaurs or rhynchocephalians. I can not accept the con- 

 tention of McGregor that the Ichthyosauria had a primitively sauro- 

 crotaphous (I need not apologize for the word) type of skull, but 

 would rather believe that they, too, enjoyed a genealogical line all 

 their own from the most primitive ty])e of reptiles, and that they 

 should no more be grouped with the dinosaurs and crocodiles than 

 with the plesiosaurs and theriodonts. 



EXPLANATION OF PLATES. 



Plate XXXIV. 



Brachauchenms lucasi, type specimen in U. S. National Museum. After Lucas. 

 (Plate from Vol. I, Quarterly, Smithsonian Misc. Col.) 



Plate XXXV. 



Skull of Brachanchenius, U. S. National Museum Collection, from Eagle Ford Shales 

 of Texas. One-fourth natural size. J5.to, exoccipital; Pf/, petrosal; *S/, stapes. 



Plate XXXVI. 



Part of front paddle of Brachauchmius. Texas speeimen, one-half natural size. 



Plate XXXVIT. 



Restored outline of skull of Brachauchenius, Texas specimen, one-half natural size, 

 ^w^;, angular; ^p, epipterygoid; Fr.?, frontal; Va.?, nasal?; /, jugal; ia, lachrymal; 

 max, maxilla; na, external naris; Pa, parietal; Pf, parietal foramen; Pfr, postfrontal; 

 Po, postorbital; Prf, prefrontal; Pm.v, premaxilla; Q, quadrate; Sq, squamosal; Sur, 

 surangular; Tv, temporal vacuity. 



