174 
Kirtley F. Mather 
the Mississippian form which is commonly identified as P. alter- 
nata (Norwood and Pratten). In comparison with the Missis- 
sippian shells the specimens in the Morrow collections display a 
much closer spacing of the concentric spine-bearing bands and 
a narrower umbonal region with a more acutely pointed beak. 
In these regards they are more closely related to the Coal Meas- 
ures form. 
When not exfoliated, these shells display an arrangement of 
the spine bases which is probably characteristic. The posterior 
half of each concentric band, with the exception of the immedi- 
ate margin, is occupied by a single row of comparatively large, 
prostrate spines, directed anteriorly ; in front of these are numer- 
ous, more closely set, smaller bases of erect spines which occupy 
the anterior half of each band, again with the exception of the 
immediate margin. Typically ten or twelve bands occur in the 
space of 20 mm. although there is considerable variation, appar- 
ently following no regular arrangement, in the width of bands 
in any individual. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Station 137). Brentwood limestone: near 
Fayetteville, Arkansas (Stations 134, 135, and 140) ; Sawney 
Hollow, Oklahoma (Station 210). Kessler limestone: East 
Mountain, Fayetteville, Arkansas (Station 209). Morrow for- 
mation: near Choteau, Oklahoma (Station 298). 
Pustula bullata n. sp. 
Plate VIII, figures 15-1 5b. 
Description. Shell below medium size, productiform, tranS' 
versely sub-oval in outline, wider than long, hinge-line not longer 
than greatest width of valve. Dimensions of the type specimen, 
an average individual, are: length from hinge-line to anterior 
margin, 10.1 mm. ; distance from umbo to anterior margin, 12.5 
mm.; length of hinge-line, ±14.5 mm.; greatest width of valve, 
15.8 mm. ; convexity of pedicle valve, ±14.5 mm. 
Pedicle valve regularly convex from beak to anterior margin 
giving a semicircular longitudinal section ; slightly flattened 
across vault of visceral region, in transverse section, with mod- 
erately steep flanks; cardinal slopes steep, approaching toward 
their base a position nearly at right angles to the direction of 
