16 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 76 



DISCOCYCLINA STEPHENSONI, new species 



Plate 6, figures 1-4 



Test flat, subdiscoid, more or less undulate, even crumpled; very 

 thin, only slightly increasing in thickness toward the center, without 

 any umbo. The diameter somewhat exceeds 4.5 mm., probably about 

 5 mm. or a little more. Thickness through the center about 0.5 mm. 

 Surface beset with very minute papillae, which measure as much as 

 40 fx thick. 



Embryonic apparatus reniform, initial chamber spherical, about 

 140ju in diameter; distance across both chambers, 240/u- 



Equatorial chambers rectangular, in definite annuli, which are of 

 about the same width, about 50ju near the center and near the 

 periphery. The length of the chambers along radial lines usually 

 exceeds the width, but some chambers are nearly square. The 

 radial chamber walls of adjacent annuli may or may not be in align- 

 ment; the latter condition appears to be the more common. The 

 equatorial zone is very thin, the actual height of the chamber cavities 

 ranging from 25/1 to about 40/x. The roofs and floors are about 25/* 

 thick. The entire thickness of the laj^er, including roofs, floors, and 

 chamber cavities, ranges from about 75/c to 90/x. The minute perfo- 

 rations leading from the equatorial to the lateral chambers are 

 excellently shown; they are small and tubular, about 1/x in diameter. 



The lateral chambers are very well defined; over the center of the 

 test they form five or six layers and near the periphery about three. 

 In places there are rather well-marked tiers. The height of the cham- 

 ber cavities ranges from about 12/i to 24/i; the length from about 40/i 

 to 100/i, the length usually measuring several times the height. The 

 roofs range in thickness from about 16/t to 32,u. There are in the roofs 

 fine perforations leading from one chamber to the next outer similar 

 to the perforations above noted for the roofs and floors of the equa- 

 torial chambers. Although the chamber ends are very well marked, 

 pillars are only slightly developed and produce small, low papillae on 

 the surface. 



Localities and geologic horizon. — From loose boulders, No. 194, can- 

 yon, about 2 miles southwest of San Pedro, and No. 196, Guerrero 

 road, about 3 kilometers east of Tanlajas, State of San Luis PotosI, 

 Mexico, collected by L. W. Stephenson, for whom the species is 

 named. Probably the Chicontepec formation, lower Eocene. 



Cotypes. — Cat. No. 371012, U.S.N.M. Four thin sections of a 

 specimen from locality 194. 



The most nearly related species is D. palenguensis Vaughan, 

 described* in this paper. That species is somewhat thicker and its 

 lateral chambers are more numerous and not so well defined. The 

 illustrations show the differences in the vertical sections. 



