Weno aud Pawpaw Formations 121 



ARCA WASHITAENSIS n. p 

 PI. 10, fig. 6 



MEASUREMENTS : I (type) II III IV V 



Length 9.2 12.6 8.1 6.7 8.0 



Height 7.0 9.5 5.0 '4.9 6.2 



Thickness 6.6 9.5 5.6 4.6 6.4 



< Length of -hinge line 7.3 9.9 6.8 5.7 7.0 



HORIZON: Pawpaw formation, clay faciea, widely distributed, John- 

 son to Denton counties ; ironstone and sandstone f acies, widely distributed, 

 Denton County to east of Bennington, Oklahoma. A similar, and prob- 

 ably identical species of Area occurs in the Grayson marl and clay at 

 Burleson and Roanoke, Texas; in the Denton clay at Denison, Texas; and 

 in the Duck Creek marl, Grayson to Johnson counties, Texas. 



LOCALITIES: 714, near Fort Worth, Texas (type locality), all Paw- 

 paw localities in the Fort Worth region. This fossil is found wherever 

 Pawpaw fossils occur, in Denton, Tarrant and Johnson counties, and 

 practically so in the Red River region. Its zone of abundance is one of 

 the most reliable markers of the Pawpaw formation. 



DESCRIPTION: Shell small, valves very inflated, subequal inequi- 

 lateral, taller posteriorly than anteriorly. Beaks prominent, remote, pro- 

 jecting, rounded, situated slightly anterior to the center of the shell. An- 

 tero-dorsal margin short, concaved; posterior margin longer and nearly 

 straight. Anterior margin short, sharply rounded, ventral margin long 

 and nearly elliptical, dorsal margin broadly rounded. Hinge line long 

 and straight and marked by a crenulated thin ridge in casts of the in- 

 terior. The shell gapes slightly at its posterior end. The valves are orna- 

 mented by numerous low, fine, subequal radiating striae, which equal in 

 width the valleys between them ; and a few (7 or more) unequal, coarse 

 concentric ridges lying mainly on the ventral half of the valve. 



This Area may be distinguished from others described from the Texas 

 Comanchean strata by its form and size. It is a very characteristic pyrite 

 and hematite fossil and is widespread geographically in the Pawpaw form- 

 ation, and in addition appears to have considerable vertical range, reap- 

 pearing in the Upper Washita in association with pyritic Engonoceras, 

 Turrilites, Hamites, Scaphites and starfishes with each invasion of the 

 clay and ironstone phase of deposition. 



