466 



THE PALEONTOLOGY OF MINNESOTA. 



LXygospirn n i rii rvi 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. headl. 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 Atrypidcp, 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 striae 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 Trenton. 



[Group I. 



Z. deflecta Hall, Trenton. 



Z. modesta (Say) Hall, Hudson Elver. 



Z. modesta, var. cincinnatiensis (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. anticostiensis Billings, Hudson River. 

 Z. headi Billings, Hudson River. 

 Z. headi, var. borealis Billings, Hudson River 



ZYGOSPIRA RECURVIROSTRA Hall, sp. 



PLATE XXXIV, FIGS. 38-41. 



1847. Atrypa recurvirostra HALL. Palceontology of New York, vol. i, p. 140, pi. xxxm, flgs. 5a-5d. 



1859. Rhynchonella t recurvirostra HALL. Twelfth Report, N. Y. State Cabinet of Nat. Hist., p. 66. 



1863. Rhynchonella recurvirostra BILLINGS. Geology of Canada, p. 168, flg. 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. 

 modesta 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. 



>// 



11 ttiicrt. Cat. Sil. FDS. Anticosti. p. 44. 1860. 



