PART E CHAZYAN AND RELATED BRACHIOPODS—COOPER 509 
Brachial valve with a deep but narrow notothyrial cavity. Brachiophores long 
and slender. Brachiophore supporting plates uniting with the median septum 
to form a cruralium. Socket plates well formed. Median septum extending to 
the anterior margin, slender and highest near the front. Septum nearly com- 
pletely dividing the interior into 2 chambers. Cardinal process a low ridge on 
the floor of the cruralium not reaching the beak. 
Genotype.—Phragmorthis buttsi Cooper, new species. 
Discussion.—The combination of characters in this genus mark it as unique. 
Phragmorthis has some features of Skenidioides and Mystrophora but is unlike 
both of them internally although the exteriors are similar. Inside the pedicle 
valve the dental plates are short and receding. The teeth are small. The umbonal 
cavities between the shell wall and the dental plates are usually filled with shell 
material. The dental plates are usually revealed as short ridges developed on the 
under side of the palintrope along the delthyrial edge. These ridges extend to 
the floor of the valve at the apex of the delthyrial cavity and form its margins. 
The muscle field of the pedicle valve is confined to the delthyrial cavity and is 
sufficiently thickened and washed onto the sides of the delthyrial cavity to make 
it appear that the dental plates and floor unite as a sessile spondylium. This is, 
however, a deception. Anterior to the muscle area a thickened patch, widest at 
its rear, extends and narrows anteriorly to reach a point not quite at the margin. 
On each side of this thickening appear the main branches of the pallial trunks 
which originate just outside and anterior to the point where the dental plates 
join the floor. No delthyrial plates of any sort were observed. 
The interior of the brachial valve is even more remarkable than that of the 
pedicle valve. The notothyrial cavity is narrow but deep and its floor is formed 
by 2 concave plates that unite on their inner sides with the top of the median 
septum. On the outside they are joined to the delthyrial edge. The median line 
of the notothyrial chamber is occupied by the slightly raised upper edge of the 
median septum, but this does not reach the beak. 
The sockets are very deep and are defined in the young by small socket plates. 
In adults the socket plates are buried by shell substance that fills in the hollows 
under the notothyrial cavity. The brachial processes are given off at the point 
where the notothyrial plates and delthyrial edge join. The brachial processes are 
long, fairly slender, and curved. On the outside they have a small thin elevated 
line which joins with the socket plate. 
The most striking feature of this genus is the median septum which extends 
from the notothyrial cavity anteriorly to the front margin. It is highest at its 
front end where it forms a blunt point. As alluded to above, the septum is con- 
tinued posteriorly as a ridge in the notothyrial cavity but does not reach the beak. 
This ridge undoubtedly serves as a cardinal process. 
The adductor muscles were located on the floor of the valve on each side of 
the median septum. They form 2 elongate scars which may be elevated on 
elongated thickenings. 
Close relationship of Phragmorthis to Skenidioides or Skenidium may be dis- 
missed because those genera have an unsupported spondylium in the pedicle 
