BRACHIOPODA. 
345 
SpiRiFERiDuE, the Athyridje, tliG Meristidje, aiicl all genera with everted brachidia 
are related to the Atryridje only through their early ancestral forms. 
The Lower Silurian faunas have furnished no evidence of species with 
everted spirals, and this hiatus in our knowledge forbids any satishictory deduc¬ 
tions as to the source or derivation of these forms. It is true in a general 
sense that the eversion of the spirals is accompanied by a convexity of both 
valves, just as the inverted spirals of the Atryridje are associated with valves 
of notably unequal depth. Still, among the latter, Glassia possesses biconvex 
valves, while of the former the group composed of Coelospira, Anoplotheca, 
Koninckina and Amphiclina, is characterized by conv^exo-plane or convexo- 
concave valves. In this group also the apices of the spirals are not directed 
toward the lateral commissures of the valves, but toward the lateral slopes of 
the pedicle-valve, such a form and direction being a necessary outcome of the 
contracted interior space. From present evidence it would seem probable that 
among the early Silurian species will be found some form whose spiral ribbon 
deviates outwardly from the vertical plane to the same degree as it inclines 
inwardly in Cyclospira and Protozyga. Indeed, in Cyclospira bisulcata itself, 
the spiral sometimes lies so nearly in the vertical plane that the inward inclina¬ 
tion of the apices is not always positive. Only some such form of the earliest 
faunas could have been the progenitor of the everted spirals. 
In the Atryrid^, possibilities of a variation in the form of the jugum were 
much restricted; in the other groups of the spire-bearers these were very great, 
and resulted in the production of a wonderful series of modifications whose 
relations it is not necessary to rehearse here. The extreme range of these 
modifications is seen in the simple termination of the jugum in Wiiitfieldella, 
Rhynchospira, etc.; the bifurcate extremity in Meristina, Eumetria and Retzia, 
their terminal branches in Kayseria, Diplospirella, etc., finally becoming co¬ 
extensive with the lamellaB of the primary spirals and thus forming a second 
pair of spiral cones. This complication of the brachidium is effected only late 
in the history of the various groups producing them. Koninckina and Amphi¬ 
clina are double-spiraled convexo-concave shells, which are the post-palseozoic 
and final representatives of Anoplotheca and Ccelospira. Pexidella and Diplo- 
