466 THE PALEONTOLOGY OF MINNESOTA. 
[Zygospira recurvi rostra. 
with a still greater number of whorls to a cone, while the loop, which is no longer 
complete in mature individuals, is placed more posteriorly than in Z. headi. In the 
Devonian specimens of Atrypa reticularis the greatest number of revolutions to a 
spiral cone is attained. The evolution of the calcified brachial supports in the 
family AtrypidcB has gone on increasing in the number of whorls to a cone, the 
connecting band has progressed from the anterior to the posterior region, and all 
has kept pace with the gradual increase in size of the various species, from the 
Trenton to the Upper Devonian. 
The species of Zygospira are divisible into two groups — (1) the depressed-convex 
species with coarse strife in which the ventral valve is more or less carinated medi- 
ally, and (2) those with the valves globose and finely striated. 
Z. RECURviROSTRA Hall — Treutou. 
Group I. 
Z. deflecta Hall, Trenton. 
Z. modesta (Say) Hall, Hudson River. 
Z. modesta, var. cineinnatiensis (James) Meek, 
Hudson River. 
Z. kentuckiensis James, Hudson River. 
Z. concentrica Ulrich, Hudson River. 
Z. paupera Billings, Anticosti. 
Z. mica Billings,* Anticosti. 
Group II, 
Z. uphami W. and S., Galena. 
Z. erratica Hall, Hudson River. 
Z. unticostiensis Billings, Hudson River. 
Z. headi Billings, Hudson River. 
Z. headi, var. borealis Billings, Hudson River 
Zygospira recur virostra Hall, sp. 
• PLATE XXXIV, FIGS. 38-41. 
1847. Atrypa recurvirostra Hall. PaUeontology of New York, vol. i, p. 140, pi. xxxiii, tigs. 5a-5d. 
1859. Rhynchonella 9 recurvirostra Hall. Twelfth Report, N. Y. State Cabinet of Nat. Hist., p. 66. 
1863. Rhynchonella recurvirostra BiLLiNGS. Geology of Canada, p. 168, fig. 152. 
1882. Anazyga recurvirostra Davidson. Supplement to British Silurian Brachiopoda, p. 129. 
Original description: "Elliptical, somewhat ovoid, very symmetrical; breadth 
about one-fourth of an inch, length a little greater; dorsal [ventral] valve with the 
middle elevated, regularly convex on the sides, the beak extended and gracefully 
incurved over the beak of the ventral [dorsal] valve, which is regularly convex, with 
a slight longitudinal depression; surface of each valve marked by about twenty-four 
regular, simple, longitudinal striae, which continue entirely to the beak." 
Minnesota examples' of this species are usually a little shorter, and therefore 
rounder than eastern examples; otherwise they are identical. Compared with Z. 
7nodesta the latter is found to attain a larger size, is more transverse and never so 
gibbous as 'this species. The beak of the ventral valve is usually less incurved, while 
the striae bounding the sinus are more prominent. Of interior characters nothing is 
known beyond the spires and the connecting band. 
*H;i!/Jidio"«''(i mica. Cat. Sil. Foss. Anticosti, p. 44, 1860. 
