174 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 83 



Genus THESCELUS Hay 



The genus Thescelus was established by Hay in 1908, and the 

 species T. insilem from the Lance formation of Wyoming was se- 

 lected as the genotype. At the same time a second species, T. rapkns^ 

 from the Ojo Alamo formation of New Mexico, was described. The 

 genus was assigned to the family Baenidae. A specimen in the 

 present collection records the second occurrence of Thescelus in the 

 Kirtland formation, and since it displays characters distinguishing 

 it from all described species, the name T. hemispherica is proposed 

 for it. 



The original characterization of the genus is no longer adequate, 

 and I therefore propose the following amended diagnosis : 



Front of carapace greatly shortened and excavated on the mid 

 line. Carapace as wide or wider than long. Fifth vertebral ex- 

 cluded from posterior border by intervention of paired supracaudal 

 scutes. Plastron extending strongly beyond the front border of the 

 carapace; bridges broad, extending far forward. Buttresses feebly 

 developed. 



THESCELUS HEMISPHERICA, new species 



Figures 11, 12 ; Plate 16 



Type. — U.S.N.M. no. 12818, consisting of the incomplete carapace 

 and plastron. Collected by George F. Sternberg, 1929. 



Type locality. — 3 miles northeast of Hunter's Store (Bisti P. O.), 

 San Juan County, N. Mex. 



Horizon. — Kirtland formation, Upper Cretaceous. 



Description. — The type specimen is an old individual as indicated 

 by the coalesced sutures, none of which is longer visible. Although 

 much of the peripheral region of the posterior half of the shell is 

 missing, a small section of the posterior rim is preserved, and it 

 permits an accurate measurement of the total length at the center 

 as being 361 mm. The greatest width at about the middle is 370 mm. 

 Since the first peripherals extend 13 mm in front of the bottom of 

 the median excavation above the neck, the total length of the cara- 

 jDace is 374 mm. Thus the relative proportions of the carapace are 

 similar to those of T. insilens from the Lance formation. 



The form of the carapace resembles the other species in being 

 broad, not greatly elevated, and having a broad median excavation 

 for the neck. It is evidently not narrowed behind as in T. insilens. 

 Tlie one figured by Wiman from which the missing parts were re- 

 stored, as shown in figure 11, has a shallow median excavation for 

 the tail. The plastron extends far forward of the line of the cara- 

 pace but otherwise resembles those of species of Baena. 



