42 BVLLETIX 16 4, UNITED STATES NATIONAL MUSEUM 



valve, 13.5 mm ; width of pedicle valve at hinge area, 19 mm ; width 

 of brachial valve at hinge area, 17.5 mm. 



Companson. — The general form of this species may be compared 

 to OrtJils calligraimna Dalman, a well-known European form, but it 

 is distinguished by the interpolation of the smaller ribs between 

 each pair of the larger ones. In its surface ornamentation it re- 

 sembles 0. intercostata Portlock, a British species, but its general 

 shape is different. 



Formation and locality. — Ordovician: Near Huang-pa-yi, Shensi, 

 China. 



<7o^2/^es.— U.S.N.M. No. 83629. 



ORTHIS CHINENSIS MULTISTRIATA, new variety 



Plate 33, Figures 9-12 



Several smaller shells associated with Orthis chinensis possess the 

 same general characters, particularlj'^ the shape, contour, and type of 

 ribbing, but the number of ribs is greater than in O. chinensis, and 

 the interspaces are much narrower. Moreover, the size of this variety 

 in general is smaller than O. chinensis, varying from 3 to 10 mm in 

 length and from 4 to 12 mm in width. 



Formation and locality. — Same as preceding. 



Cotypes.—V.S.'^.lsL No. 83630. 



ORTHIS SHENSIENSIS. new species 



Plate 34, Piguees 1-S 



Description. — Shell plano-convex, subquadrate, rounded to semi- 

 elliptical; hinge line rather longer than or equal to width of shell; 

 cardinal angle rectangular. Pedicle valve convex, swollen; beak 

 rather high, incurved. Brachial valve flattened or slightly convex 

 with comparatively weak median depression widening anteriorly; 

 beak small, inconspicuous, not raised. 



Interior of pedicle valve with isosceles triangle muscle scar, less 

 clearly defined, composed of two narrow diductors inclosing ad- 

 ductors of about one-half the width of the scar. 



Surface of valves ornamented with about 40 rounded and com- 

 paratively weak ribs of equal strength, which are spaced equidistant 

 along the margin; about half the number of primaries, the others 

 being intercalated in the interspaces or branching off the primaries 

 at about one-fifth to one-half their length. Interspaces about the 

 same as the width of ribs. Ribs and interspaces crossed by com- 

 paratively strong concentric lines. Ribs curve back slightly on 

 both sides toward cardinal angles. 



The dimensions of nearly complete, medium-sized specimen (pi. 

 34, figs. 2 and G) are: Height of pedicle valve, 7.5 mm; height of 



