PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 135 
cialized to the extent that it has lost the pseudodeltidium. The stock is an ancient 
one, first appearing in rocks laid down in Marmor time, and extends into the 
late Porterfield stage. 
KULLERVOIDAE.—This family is represented by a few specimens only of the 
genus Kullervo. Pseudopunctae, which are a family character, were not seen in 
the American shells. 
CAMERELLIDAE.—After the Whiterock stage, members of the Syntrophiacea 
are rare or absent, but the Camerellidae take their place and are often very 
abundant. The group reaches its zenith in the rocks deposited from Whiterock 
to Wilderness time. After that time the group declines. Certain trends may 
be detected in some genera. Plectocamara is especially noteworthy for the reduc- 
tion of the median septum of the brachial valve. Brevicamera, which forms a 
family by itself, is noteworthy because of its sessile cruralium, the only genus to 
achieve such a structure. The variety of exteriors and shapes exhibited by mem- 
bers of the Camerellidae may ultimately permit further separation of genera. The 
camerellids are structurally like syntrophids and probably arose from them, but 
the change from a spondylium simplex in the latter group to a spondylium 
duplex of the Camerellidae cannot be explained. 
PARALLELELASMATIDAE.—This family belongs to the Pentameracea and is 
characterized by having two long, subparallel septa in the brachial valve which 
enclose the muscle scars. The family is represented by three genera unlike in 
appearance but close internally. Parallelelasma occurs in the Pratt Ferry forma- 
tion and is the earliest known pentamerid. Although it is an early form, it is 
nevertheless a very complicated shell in its articulation at both ends of the valves. 
The webbed costae at the front are a novel strainer device. The source of this 
unusual group was not detected in the early Middle Ordovician; it must be 
sought in Whiterock or earlier faunas. 
RHYNCHONELLACEA.—One of the important events that took place in the time 
spanned by this monograph is the arrival of the rhynchonellids. These appear in 
the form of Rostricellula and Ancistrorhyncha. The former is the earlier, but 
it is the more complicated shell of the two. Rostricellula has the standard rhyn- 
chonellid complement of a cruralium (or septalium), hinge plate, and strong 
median septum in the brachial valve, and dental plates in the pedicle valve. 
Ancistrorhyncha, on the other hand, has complicated crura, a divided hinge plate, 
but no median septum. This is probably an advanced form which cannot be an- 
cestral to the rhynchonellids. The same is true of Rostricellula which is an early 
but specialized type. The ancestor of the group must be looked for in rocks de- 
posited in Whiterock time or earlier. Several small shells, such as Oligorhynchia, 
Dorytreta, and Sphenotreta, appear to be rhynchonellids. These are probably 
primitive forms in that they have a fold on the pedicle valve, which is an early 
character. They are, however, specialized forms, too, in their complicated orna- 
mentation, especially Oligorhynchia. 
ATRYPACEA and SpiIRIFERACEA.—The advent of the calcified lophophore, as 
far as now known, took place within the time embraced by this monograph. The 
shells are all small and are generally rare in the earlier formations. The first 
