BRACHIOPODA. 157 



modesta, (Say) Hall, Atrypa recurvirostra* and A. dejleda, Hall. In the Hudson 

 River group are the species Z. modesta, (Say) Hall, Z. Kentuckiensis, James, 

 Z. Cindnnatiensis, Meek, Z. concentrica, Ulrich, Z. pau- 

 pcra, Billings, and probably the Rhynchonella mica, 

 Billings. There is still another species in this fauna 

 as it is developed in Pike county, Missouri, Zygospira 

 putilla, sp. nov. (see Plate LV, figs. 35-37), which 

 possesses an unusually elongate form, but retains 

 the coarsely plicate surface of Z. modesta ; its loop 

 appears to be persistently posterior in its posi- 

 tion. ,10 i>. 



T r> pi.ii ±^ ' 1*1 Preparation showing tlie brachidium 



In faunas 01 later date occurs the species which or zygosptra putaia. (c) 



has been described as Z. minima, Hall, in the Niagara group at Waldron, Indiana, 

 but it is exceedingly rare, and its internal structure is not known. The genus 

 has not been satisfactorily identified in European faunas. 



Subgenus CATAZYGA, s.-gen. nov. 



PLATE LVI. 



Mr. E. Billings described,! in 1862, the species Athyris Headi, from the Hud- 

 son River formation on the " south shore of the St. Lawrence, opposite Three 

 Rivers." It is a rather large, subcircular or ovoid shell, with valves more con- 

 vex than in Zygospira, the rotundity of the pedicle-valve obscuring the usual 

 prominence of the umbo in that genus. Both valves bear a low median sinus, 

 while the external surface, instead of being coarsely plicated as in Zygospira, is 

 covered with a great number of fine radiating striae. The typical external expres- 

 sion of Zygospira is thus to a large degree lost. On the interior of the pedicle- 

 valve the muscular impressions are well defined and similar to those seen on the 

 internal casts of the Orthis? or Zygospira erratica, from the sandy Hudson River 



* Messrs. Winchell and Schuchert have recently separated fi'om the shells usually _i-eferred to this 

 species certain lai-ger and more finely striated shells from the Trenton and Galena horizons. These are 

 \.nYmeCL Zygospira Uphami. See American Geologist, vol. ix, p. 291 (1892), and Geological Survey of Min- 

 nesota, vol. iii, p. 468. pi. xxxiv, figs. 45-48 (1893). By the favor of Prof. N. H. Wincuell we have been 

 permitted to refer to advanced pages of the latter work. 



+ PaliEozoic Fossils, vol. 1, p. 147, fig. 125. 



