132 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
knowledge of the shell structure, and furthermore the problem is one that con- 
cerns all the brachiopods and not just those of the early Middle Ordovician. 
NOTES ON EVOLUTION 
The time in which the brachiopods here described lived is not a long one geo- 
logically speaking, yet considerable change took place in brachiopod evolution. 
During this time the appearance and rise of several major groups, as far as now 
known, took place, namely, the dalmanellids, rhynchonellids, and spiriferoids. 
In addition this time saw the heyday of the plectambonitids. Among the Gastro- 
caulia, Cambrian types still lingered on, but the group as a whole had already 
lost ground to the Pygocaulia except in a few limited habitats. 
Gastrocaulia.—Perhaps the most interesting feature connected with the in- 
articulate brachiopods is the persistence into the Middle Ordovician of genera 
that definitely have Cambrian affinities. This is strikingly seen in the genus Dic- 
tyonites which is structurally a member of the Dictyoninacea, new superfamily, 
a specialized group of early brachiopods. This group at present defies classifica- 
tion in either Neotremata or Atremata, because there is no pedicle foramen, nor 
is there a pedicle groove as in the linguloids. 
The appearance of Lingula in the Ordovician has been stated as a fact, yet 
the writer does not use that name for any of the linguloids described herein. 
The etched linguloids from Pratt Ferry make it clear that, on hinge structures 
at least, the linguloids of the part of the Ordovician covered by this monograph 
are varied and some of them have the definite characters of Lingulella. It can- 
not now be said that Lingula did not exist in the Middle Ordovician, but the 
specimens that could be prepared, and the apical structures revealed, all proved 
to have other affinities. For this reason the name Lingulella was preferred to 
Lingula for species whose apical structures could not be revealed. 
The limestone of the Pratt Ferry formation in particular and bituminous lime- 
stones associated with black shales in general yielded many specimens of the 
conical Acrotretacea. These showed more variety than has hitherto been re- 
vealed in the Middle Ordovician. The genus Conotreta is obviously related to 
the Cambrian shells now called Homotreta and others like them, but the Ordo- 
vician genera seem to have more elaborate pallial markings than those from the 
Cambrian. The bizarre interior of Ephippelasma is still not understood, but it 
is here suggested that the saddle-shaped plate of the brachial valve is probably 
a lophophore support. 
Lingulasma is a giant form of linguloid characterized by elevated muscle plat- 
forms. The genus seems to be totally unrelated to the Trimerellidae but may 
have affinities with Lingulops. 
The early trimerellid genus Obolellina is characterized by the modest develop- 
ment of the muscle platforms which become highly elevated in the Silurian mem- 
bers of the superfamily. 
The genus Schizambon is of interest because it is a spiny genus that appeared 
in the earliest Ordovician and persisted into late Middle Ordovician. The mem- 
