154 
Kirtley F, Mather 
situated at intervals of about 1.4 mm. from center to center ; 
towards the cardinal line the rows of spines are bordered by 
a faint irregular ridge which is parallel to them and bounds a 
non-costate area which includes the lowermost cardinal slopes 
and the posterior portion of the main flanks and extends across 
the ears to the cardinal extremities. This part of the valve, 
which is rarely preserved, is marked only by concentric ribs 
and lines of growth. 
Brachial valve very gently concave or nearly plane through- 
out the visceral portion but strongly geniculated toward the 
anterior and lateral margins ; hinge-line straight and long, beak 
not elevated, cardinal extremities somewhat auriculate. Sur- 
face markings the same as those of pedicle valve, with the 
concentric ribs limited to the gently concave visceral portion 
of the valve. 
Remarks. This, the most frequently collected and most char- 
acteristic of all the Morrow fossils, is a form which although 
obviously closely related to P. semireticulatus is readily recog- 
nized by the peculiar arrangement of spines on the cardinal 
slopes and main flanks of the ventral valve. In view of the 
persistence of this character the variations in depth of sinus, 
curvature of valve, and outline and size of shell, which may 
be observed, assume a minor importance and an attempt to 
subdivide the group on any of those bases seems to be futile. 
All the specimens from the Kessler limestone are below the 
average size of the species and the form is not nearly so abund- 
ant in that formation as in the two lower fossiliferous horizons. 
Horizon and locality. Hale formation: vicinity of Fayette- 
ville, Arkansas (Stations 136 and 149). Brentwood limestone: 
vicinity of Fayetteville (Stations 135, 138, 140, 147, and 150), 
near Brentwood (Station 145), and near West Fork (Station 
154), Arkansas; Sawney Hollow, Oklahoma (Station 210). 
Kessler limestone: near Brentwood (Station 144), and on East 
Mountain, Fayetteville (Station 209), Arkansas. Morrow for- 
mation: near Tahlequah (Station 293), Ft. Gibson (Station 296), 
Choteau (Stations 297, 298, 302, 306, and 307), Hulbert (Sta- 
tion 299), and Gore (Station 305), Oklahoma, 
