PART I CHAZYAN AND RELATED BRACHIOPODS—-COOPER 507 
TROPIDOTHYRIS Cooper, new genus 
(Greek tropis, keel; thyris, opening) 
Shell small to minute, subpentagonal in outline ; hinge forming the widest part ; 
sides oblique; anterior margin narrowly rounded; anterior commissure deeply 
sulcate ; brachial tongue long and acute; surface smooth to more or less plicate, 
the plicae when present not always reaching the front margin. Impunctate. 
Pedicle valve with long apsacline interarea and open delthyrium ; teeth small ; 
free spondylium deep and long. 
Brachial valve with deep and narrow notothyrial cavity bounded by subparallel 
brachiophore plates attached to the valve floor ; brachiophores long and slender ; 
sockets defined by small fulcral plates ; cardinal process long and slender, strongly 
elevated within the notothyrial cavity ; median septum long and slender, strongly 
elevated at the anterior end to form a partition dividing the valves. Adductor 
impressions located on each side of the median septum. 
Genotype.—Tropidothyris pentagona Cooper, new species. 
Discussion.—This interesting little genus is undoubtedly related to Skenidioides 
and is probably a lateral branch of it. The resemblance is to be seen in outline 
and folding. Although a resemblance between the two genera is easy to see, 
nevertheless some important exterior differences occur in the ornamentation. 
The ornamentation of Skenidioides consists of narrowly rounded, usually direct 
or implanted costae; some specimens of Tropidothyris may be quite smooth. 
Others are marked by broad plications often indistinct and often not reaching the 
margins of the shell. Furthermore, although the folding and sulcation of the two 
genera are of the same type, those of Tropidothyris are much more strongly 
developed. 
The interior of the valves of the two genera also show differences while at the 
same time showing close relationships. The free spondylium of the pedicle valve 
of Tropidothyris is usually deeper than that of Skenidioides probably because its 
valve is deeper and narrower. In the brachial valve the chief generic differences 
occur. In Skenidioides the notothyrial chamber is generally fairly wide and shal- 
low and the brachiophore plates are attached to the medium septum. When 
viewed from the anterior these plates are distinctly attached to the valve floor by 
callus or elevated above the floor. This is clearly the case in S. billingst Schuchert 
and Cooper, the genotype. The brachiophore plates of Tropidothyris on the 
other hand, do not unite with the median septum but are attached to the floor of 
the valve. They make a subparallel thickening along the floor on each side of the 
shaft of the cardinal process, the anterior ends often swinging toward the median 
septum to close off the notothyrial cavity. The median septum and cardinal 
process are like those of Skenidioides. 
This little genus is known only from the Pratt Ferry locality in Alabama. 
TROPIDOTHYRIS PENTAGONA Cooper, new species 
Plate 99, A, figures 1-14 
Shell small, subpentagonal in outline and with the hinge forming the widest 
part; cardinal extremities acutely rounded; sides gently rounded, oblique; an- 
