926 Report oF THE StatE GEOLOGIST. 
brachidia are related to the Arryrmpz 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 satisfactory deductions 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 ArryPip# are associated with valves 
of notably unequal depth. Still, among the latter, Guassta 
possesses biconvex valves, while of the former the group composed 
of Catosprra, ANopLorHeca, Konrnexina and AmpuHictina is 
characterized by convexo-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 Cycrosrira and Protozyea. Indeed, in Cyclospira 
bisulcata itself, the spiral sometimes lies so nearly in the vertical 
plane that the inward inclination 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 Avrrypm possibilities of variation in the form of the 
jugum were much restricted; in the other groups of the spire 
bearers they were very great, and resulted in the production of 
a wonderful series of modifications whose relations it is not neces- 
sary to rehearse here. The extreme range of these modifications 
is seen in the simple termination of the jugum in WuitFIELDELLA, 
RaynowospirA, etc.; the bifurcate extremity in Merisrina, Eume- 
rrra and Rerzra, these terminal branches in Kaysrria, Dreospr- 
RELLA, etc., finally becoming coextensive with the lamelle 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. Konyrnoxrna and 
Ampatoiina are double-spiraled convexo-concave shells which are 
the post-palzozoic and final representatives of AnopLoruEca and 
Catosprra. Prxipenta and Dretosprretia, of the St. Cassian 
178 
