344 
PALAEONTOLOGY OF NEW YORK. 
shells, the jiigum, however, which is subject to the most frequent variations in 
form, and which serves as the generic index. When the spirals are directed 
outward toward the lateral margins of the 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 lamellas than occurs in the former. 
The earliest spire-bearing shells yet discovered are the simplest in the 
structure of the brachidium. Hallina, Protozyga, Cyclospira, of the Lower 
Silurian, possess brachidia which make a little less than one or two volutions 
of the calcified lamellae, with a slight inclination toward each other, and to the 
median axis of the shell. Zygospira and Glassia, the contemporaries and suc¬ 
cessors 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 transY^erse 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 Atrypa, 
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 revo¬ 
lution 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 to this type {AtrypidW). 
It is well to emphasize the fact, lest misc.onceptions 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 Cyclospira and Spirifer, but the spirals have never, by gradual 
changes, revolved from their inverted position m 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 lamellm and 
jugum on the dorsal side as in all shells with everted spirals, these parts would 
lie on the ventral side of the shell. It must hence be inferred that the 
