74 
Kirtley F. Mather 
in which it occurs in abundance in the vicinity of Prairie Grove 
and elsewhere in northwest Arkansas. The latter species is not 
known to occur beneath the Hale formation and is one of the 
most characteristic of all the Morrow brachiopods, being pres- 
ent in great numbers in each of the fossiliferous horizons of the 
group. A third species of Dielasma, not previously known, is 
associated with these two. Another small Dielasmatinae rep- 
resents a new species and probably a new genus. It is here 
referred to Girtyella, a genus known at present only from Mis- 
sissippian strata, as it appears to be most closely related. 
The Spirifeyndae include some of the most characteristic and 
abundant species of the fauna. Spirifer is represented by three 
species. S. opimus, long relegated to the synonomy of the next- 
named form but believed to be a distinct species, is known from 
the Coal measures of Ohio and Iowa, while S. rockymontanus 
has a wide distribution in the Pennsylvanian limestones of the 
west. In the Mississippi valley, where it has been reported 
from the Coal Measures of half a dozen states, it seems more 
characteristic of the lower than of the upper portion of the 
series. In Ohio and Pennsylvania it occurs in the Mercer and 
Vanport limestones. S, goreii, a new species, is suggestive of 
S. striatus of the Mississippian and may be closely related to 
it. Brachythyris, represented here by a newly described form, 
has not been previously reported from post-Chester strata in 
North America but it is noteworthy that several of the spirifers 
described by Tschernyschew from the Upper Carboniferous of 
the Ural and Timan Mountains are apparently to be referred 
to this genus which many consider to be a subgeneric group 
under Spirifer, Squamularia perplexa is fairly abundant in 
the Morrow and has much the same distribution and range as 
Spirifer rockymontanus. In the Kansas section it is restricted 
to Stages A to G, not occurring in Series IV at the top of the 
system. Associated with it is a larger undescribed form. The 
genus is characteristic of the Pennsylvanian and is not known 
to occur in the Mississippian formations of this continent, 
Spiriferina is represented by two species, S. campestris, typ- 
ical of the Pennsylvanian of the west and southwest, and S. 
transversa, a Chester form, reported from the Coal Measures 
of Brazil and apparently replaced in the mid-Pennsylvanian seas 
