PART jt CHAZYAN AND RELATED BRACHIOPODS—-COOPER 613 
is the slender fold of the shell on the outside of the tooth, which leaves a narrow 
slot. This fits against the angulated edge of the brachial valve. No trace of 
muscle marks was seen in the spondylium. Narrow interareas appear on each 
side of the delthyrial cavity. In old specimens the edge of the shell from the 
interarea anteriorly for some distance is flattened and slotted to take the edge of 
the opposite valve. 
The interior structures of the brachial valve are of considerable interest. These 
have a strong resemblance to the cardinalia of Pentamerus and allies, but a num- 
ber of important differences can be seen. A long and deep socket divides the out- 
side wall of the shell from the brachiophore. The base of the socket is defined 
by a deeply concave fulcral plate. The brachiophore is a flat, short, sharply 
pointed straight blade bounding the socket and, by its progressive growth, form- 
ing a ridge bounding the notothyrium. The outer edge of the brachiophore is 
thickened along the socket wall, and opposite the end of the socket it terminates 
in a fairly long triangular tooth. This process fits on the inside of the tooth of 
the pedicle valve and helps to keep the two valves fitted in place. The brachio- 
phore supporting plates are high and slender but short, being confined to the 
posterior quarter or less of the valve. Not one specimen in the collection showed 
any trace of the muscle field. However, low ridges between the brachiophore 
plates on a few specimens suggest that the muscles were confined to the noto- 
thyrial cavity. 
A structure of interest in the brachial valve is the small ridge on each side of 
the beak at the posterior of the valve. This extends obliquely from the beak to 
the posterior termination of the socket. It overlies and crosses the posterior end 
of the brachiophore. Its use to the brachiopod is not immediately apparent. 
The chief similarity between Parallelelasma and Pentamerus is the presence 
in each of the nearly parallel brachiophore supporting plates. Here the simi- 
larity ends because the cardinalia of the Ordovician genus are not divisible into 
inner and outer plates like the cardinalia of Pentamerus. The inner plates of the 
latter produce a broad divided hinge plate and shallow notothyrial cavity. Fur- 
thermore, the brachial process is given off at the anterior end of the vertical plate 
deep inside the valve. Similar difficulties confront a comparison with members 
of the Gypidulinae. 
The exteriors of the Pratt Ferry genus and Salonia are totally unlike except 
in the beak region and the compressed marginal areas. Inside the brachial valve 
the similarity consists, in the main, of the presence of two long and subparallel 
plates. The cardinalia are actually not very close. The important difference be- 
tween them is in the length and form of the brachial process, which is long and 
expanded in Salonia but short and straight in Parallelelasma. This feature also 
constitutes the chief difference between the Alabama genus and Metacamarella 
from the Girvan District, Scotland. 
The exterior of Metacamarella is very similar to that of Parallelelasma, but 
sufficient characters are known to establish the two as independent genera. The 
exterior of Metacamarella is paucicostate like that of the Alabama shell, but the 
costae are more concentrated in the median region to form a low, indistinct fold 
