BRACHIOPODA. 467 



Zygospira modesta.l 



In the nepionic stage of this form, in specimens about 1 mm. in length, the shell 

 is depressed-convex without striations and plications, the beak of the ventral valve 

 being erect and perforated by a large triangular delthyrium. This 1 stage agrees 

 essentially with the same age in species of Rhynchonella* and Rhynchotrema incequi- 

 valvis. In some individuals of Z. recurvirostra the plications begin to develop along 

 the anterior margin much earlier than in others. During the succeeding stages of 

 growth the valves attain greater gibbosity, the delthyrium of the ventral valve 

 becomes partially closed by the deltidial plates, the beak incurves over that of the 

 dorsal valve and the striae become larger until a certain size is reached, after which 

 new ones are introduced maintaining their equality. 



Z. uphami appears to be a descendant of Z. recurvirostra. It differs in having 

 attained a larger growth and greater convexity. The striae, however, do not increase 

 in size, but numerous new ones are added, so that Z. uphami appears more finely 

 striated. 



Formation and locality. This species occurs throughout the Trenton shales, but is very abundant 

 near the base of the Galena shales in association with Phelidops trentonensis, var. minor, Orthis pectinella, 

 var. siveeneyi, and Rhyncholrema intequivalvis at Minneapolis, St. Paul, Cannon Falls and Fountain, Min- 

 nesota. Also common in the Galena south of Cannon Falls and Kenyon, Minnesota, and at Oshkosh, 

 Wisconsin. Near the top of the Trenton in association with Orthis borealis Billings, at Lexington, Dan- 

 ville and Frankfort, Kentucky. Martinsburgh, Lowville and Middleville, New York; Ottawa, Canada. 



Collectors. C. L. Herrick, W. H. Scofleld and the writers. 

 Mus. Reg. Nos. 437, 439, 735, 767, 4069, 5477, 5511, 8220-8223. 



ZYGOSPIRA MODESTA (Say) Hall. 



PLATE XXXIV, FIGS. 42-44. 



1817. Atrypa modesta HALL. Palaeontology of New York, vol. i, p. 141, pi. xv, flg. 15. 



1859. Genus ? modesta HALL. Twelfth Annual Eeport, N. Y. State Cabinet of Natural History, 



p. 66. " Belated to Leptoccelia." 



1860. Atrypa modesta HALL. Ibidem, Thirteenth Eeport, p. 69. 



1862. Zygospira modesta HALL. Ibidem, Fifteenth Report, p. 154. 



1863. Rhynchonella t modesta BILLINGS. Geology of Canada, p. 211, flg. 211. 



1867. Zygospira modesta HALL. Twentieth Eeport, N. Y. StateCab. of Nat. Hist., p. 267, figs. 1, 2. 



1873. Zygospira modesta MEEK. Paleontology of Ohio, vol. i, p. 125, pi. xr, flg. 4. 



1875. Zygospira modesta MILLER. Cincinnati Quarterly Journal of Science, vol. ii, p. 58. 



Original description: " Suborbicular or plano-convex, with the beak extended; 

 width a little greater than the length; cardinal line distinctly marked and somewhat 

 extended; dorsal [ventral] valve convex, with an elevated ridge along the center, 

 occupied by four plaits which are stronger than the others; beak prominent, incurved 

 and perforated, the perforation [pedicle opening] extending below the beak and 

 occupying a portion of the area; ventral [dorsal] valve depressed-convex, broadly 

 oval or nearly circular, with a broad but ill defined sinus along the middle, the 

 central plication stronger than the others, with a smaller one on each side; each 



*The Development of some Silurian Brachiopoda. By Beechft- and Clarke. Mem. N. Y. State Mus., vol. i, no. 1, 1889. 



