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 



Fio. 189. Pio. 140. Fig. 141. 



Preparations showing tlic strnctare of the brachidium In Baltina Saffordi, Winchell and Sohnchcrt. 

 Fig. 139. The pedicle-valTe cat so as to show the tips of the ascending lamellie. 



T\g. 140. The opposite side, showing the form of the primary lamella as far as the base of the loop, and the char- 

 acter of the latter. 

 Fig. 141. View showing the form of the bracbidiam 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 

 ZyaospiBA, 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 Protozyoa 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 



* BalliTui 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 oi Atrypa biaulcata had been demonstrated and described in manuscript under 

 the name Ctclospiba, some time before the treatise on the Silurian Brachiopoda of Minnesota, by Winchbll 

 and ScHDCHBRT (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 Ctclospiba was there used with our knowledge and consent. ' 



