i6 



AMERICAN MESOZOIC MAMMALIA 



Ctenacodon and Plagiau- 

 lax are closely related and 

 have, indeed, sometimes been 

 considered to be synonymous. 

 It has elsewhere been shown, 

 however, that Ctenacodon ser- 

 raius and Plagiatdax becklesii 

 belong in different genera 

 (Simpson 1928B). Plagiaulax 

 has no Pi, P2 is relatively 

 smaller and simpler, the ridges 

 on the shearing teeth extend 

 far down on the sides, and the 

 jaw is relatively shorter and 

 stouter. "Plagiaulax" minor 

 Falconer and "P." falcoiteri do 

 not belong to Plagiaulax, as 

 was first recognized by Cope 

 who placed them in a new 

 genus, Pliofrion. Pliofrion 

 proves, however, to be insepa- 

 rable from Ctenacodon, which 

 thus includes four species based 

 on lower jaws. The two Eng- 

 lish species, Ct. minor and Ct. 

 falconeri resemble one another 

 somewhat more closely than 

 they do the American species, 

 and the same is true of the lat- 

 ter, Ct. serratus and Ct. scin- 

 dens, with respect to the Eng- 

 lish species. P4 is larger rela- 

 tive to P3 in the English species 

 and they also have a slightly 

 greater number of serrations on the shearing edges of these teeth, apparently indicat- 

 ing that these forms are somewhat more advanced than the American ones. 



Plagiaulax becklesii 



Fig. 4. Ctenacodon and Plagiaulax. Comparison of American 

 and European species. Right lower jaws, external views. Three 

 times natural size. (From Simpson, 1928B.) 



Ctenacodon serratus Marsh 1 879 



1879. Ct. serratus, Marsh, Amer. Jour. Sci. (3) XVIII, 396. 

 1881. Ct. nanus. Marsh, Amer. Jour. Sci. (3) XXI, 512. 



Type. — Cat. No. 1 1833, Yale Peabody Museum. Right lower jaw, free of matrix, 

 lacking M2 and extreme anterior and posterior parts of ramus. Fig'd, Marsh 1887, PI. 

 VIII, fig. I. 



