BRAcuHIOpoDA. 925 
valves, the jugum seems to be much more variable than in shells 
where the spirals are introverted or take some intermediate 
position. In the latter there is a much greater variation in the 
position of the loop upon the primary lamell than occurs in 
the former. 
The earliest spire-bearing shells yet discovered are the simplest, 
in the structure of the brachidium. Hatiina, Pro ozyea, 
Cyctospira of the Lower Silurian possess brachidia which make 
a little less than one or two volutions of the calcified lamelle, 
with a slight inclination toward each other, and to the median 
axis of the shell. Zyespira and Grassta, the contemporaries 
and successors of these primitive structures, show. progressed 
conditions of the same form of brachidium. In these genera, 
however, there is a slight deviation in the vertical axes of the 
spirals from the transverse axis of the shell, the apices being 
inclined somewhat toward the brachial valve, and this tendency 
to lateral evolution in the spiral cones is carried to its extreme in 
the genus A rypa where the multispiral cones of the fully 
matured forms of the Devonian, may sometimes have their axes 
nearly parallel. This is the termination of all revolution of the 
cones, a change through an arc of less than 90°, probably due 
in a large degree to alterations in the form of the internal cavity 
of the valves; and the fact that this revolution here ceases, 
strictly delimits the group of forms bearing spirals of this type 
(ATRYPID2). 
It is well to emphasize the fact, lest misconceptions already set 
on foot should become prevalent, that no wider revolution of the 
spiral cones exists. It is true that there is a difference of 180° 
in the position of the axes of the spiral cones in CycnospirA and 
Sririrsr, but the spirals have never, by gradual changes, revolved 
from their inverted position in the former to their everted 
position in the latter. Such a process might have been possible, 
but had it actually occurred the forms resulting would have been 
totally different in structure from any now known. Instead of 
having the primary lamelle and jugum on the dorsal side as in 
all shells with everted spirals, these parts would lie on the ventral 
side of theshell. It must hence be inferred that the Sprrrrzrms, 
the Ar#rrips, the MerisTip, and all genera with everted 
V7 
