BRACHIOPODA. 
151 
horizon, the Atrypa, Zygospira or Anazyga recurvirostra. Hall. The value of 
the genus Hallina must be derived from this species, and, after examination 
Fig. 139. Fig, 140. Fig. 141. 
Preparations showing the structure of the brachidium in Hallina Saffordi, Winchell and Schuchcrt. 
Fig. 139. The pedicle-valve cut so as to show the tips of the ascending lamellae. 
Fig. 140. The opposite side, showing the form of the primary lamell® as far as the base of the loop, and the char¬ 
acter of the latter. 
Fig. 141. View showing the form of the brachidium in profile. (c.) 
of specimens from the original locality, we have been unable to find evidence 
that it is any more terebratuloid in its characters than the Hallina Nicolleti; 
indeed, it possesses a brachidium of precisely the same structure as the latter.* 
The Atrypa exigua and Hallina Saffordi present the minimum development of 
the spiral cones; the inward inclination of their apices, though but slight, and 
the highly developed loop, show that they are actually inceptive forms of 
Zygospira, while the difference in external surface of the two, smooth in the 
former except for the low folds about the margins, finely and completely plicated 
in the latter, the nearly vertical plane of the spirals, as well as their brevity, 
afford again evidence of the great variability in early types of structure. For 
the Atrypa exigua the term Protozyga is proposed; its relations to Cyclospira 
are evident, the differences between the two lying in the longer, more nearly 
vertical and parallel spirals of the latter, and (with the present evidence) in 
its incomplete loop, indications only of jugal processes being present near the 
posterior part of the primary lamellae, f 
* Hallina Saffordi has a simple hinge-plate composed of two discrete processes, upon which the crura 
are based, a low median septum in the brachial valve, and well defined though small dental plates. For 
further illustration of this shell, see Supplementary plate, 
t The internal structure of Atrypa bisulcata had been demonstrated and described in manuscript under 
the name Cyclospira, some time before the treatise on the Silurian Brachiopoda of Minnesota, by Winchell 
and ScHUCHERT (Geological Survey of Minnesota, vol. iii) was undertaken. As it proved desirable to refer 
to this type of structure in that work, and as the determinations_above given, were known to one of the 
authors, the name Cyclospira was there used with our knowledge and consent, ’ 
