beede: upper per mi an red beds. 153 



will be noted later, is prominent on some of the shells of this 

 group. 



So far as can be seen from our casts, the posterior adductor 

 scar is large, and situated just above the umbonal ridge, op- 

 posite the extremity of the posterior tooth. 



The form of the right valve differs from that of the left 

 only in usually possessing a slight sinus in the ventral margin 

 beneath the beak. The posterior end of the hinge is armed 

 with two horizontal teeth instead of one. The anterior 

 dentition is such as to correspond with the other valve. The 

 surface of the shell seems to be marked with lamellar growth 

 lines, arranged closely along the hinge and more remotely on 

 the other parts. Length of hinge, '5. 5 mm. ; length of shell, 

 9.5 mm. ; height at posterior end of shell, 5 mm. 



Whitehorse spring, Oklahoma; common to abundant. 

 Dozier, Tex. ; common to abundant. 



In some of the right valves of this species the buttress 

 seems to be disconnected from the third tooth. 



CYRTODONTARCA ? MUNTIDENTATA, n. sp. 



Plate VI, figures 4, 4a, 



Shell practically equivalvular, left may be a trifle the more 

 convex, the form similar to the preceding species. There is 

 a very slight sinus beneath the beak. The hinge has five 

 teeth in the anterior end of the right valve and six in the left, 

 with vertical buttress in addition. The posterior end of the 

 hinge is furnished with two horizontal teeth to each valve. 

 It may be, however, that the upper impression shown in the 

 cast is a ligamental impression. The upper one is the more 

 prominent of the two, the lower being more remote from the 

 beak and smaller, and appearing only on the maturity of the 

 shell. Length of hinge, 6 mm. ; length of specimen, 10 ram. ; 

 height at posterior end, 5 mm. 



Whitehorse spring, Oklahoma ; rare. Dozier, Tex. ; rare. 



CYRTODONTARCA ? PARALLELIDENTATA n. sp. 



Plate VI, figures 3-3c. 



This species is more closely related to BakeweUia parva 

 Meek, than any other species of the Whitehorse beds. It 

 has a much closer relative, however, in an undescribed 



