'2(>4 VUOCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



PLECTORTHIS IDDINGSI, new species. 



General form and convexity much like that of yome varieties of P. 

 ifullanola. Its surface characters also approach those of the more 

 regularly ribbed shells of the latter species. The principal diti'erence 

 is in the less convi'xit}', lower um])o, and lower area of /*. nldrngs'i. 

 'I'he interior of the ventral valve shows a short, well-defined umbonal 

 cavitv, with a strong- vascular sinus on each side of it that extends well 

 forward into the valve. 



The species is named after Prof. Joseph P. Iddings of the Univer 

 sity of Chicago, formerly of the U. S. Geological Survey, who col- 

 lected the specimen. 



Fo I' Illation and locality. — Upper Cam))rian. Gallatin terrane, south 

 side of Gallatin Valley, Yellowstone National Park, Wyoming. 



PLECTORTHIS INDIANOLA, new species. 



vShell small, transverse, with the cardinal extremities rounded, angu- 

 lar, and in some specimens almost alate. Valves moderatel} convex, 

 the ventral nearly twice as much so as the dorsal; the hinge line Aaries 

 in length from slightly less than the greatest width of the shell to 

 where, in the shells with extended cardinal extremities, it is one-tifth 

 greater than the width of the shell across the center. 



The ventral valve may or may not have a mesial depression. In 

 some examples the depression is broad and rounded, and in others it 

 is shallow with stronger ribs at each margin; umbo somewhat elevated, 

 and curving over toward the hinge line, beyond which it projects, ter- 

 minating in a small, incurved beak; dorsal valve considerably less 

 convex than the ventral; beak small and projecting slightl3^ over the 

 hinge line. The median sinus in the dorsal valve varies from a broad, 

 shallow depression to a narrow, rather deep furrow that gives a 

 bilobed appearance to the valve. 



The surface is quite variable in the strength and arrangement of the 

 strong and minor radiating elevated ribs and striae. On some shells 

 the}^ are very tine and of nearly equal size, separated by grooves of 

 about the same width as the ribs or striae. In other examples every 

 third or more widtdy separated rib is stronger and elevated above a 

 more or less broad interspace marked by line ribs or costte. 



Almost nothing is known of the interior of the valves. A few 

 specimens show a well-marked umbonal cavity beneath the umbo that 

 is nmch like that of O. renin icha. The area of the ventral valve is 

 inclined backward at an angle of about 65° to the plane of the margin 

 of the valve. It is rather low and broken midway by a strong delthy- 

 rium. The area of the dorsal valve is low and inclined well out over 

 the hinge line. 



Ohservations. — Plectorthis indianola is almost as variable in its form 

 and surface markings as P. wichitaensis^ with which it is associated at 

 a luuiibcr of localities. It differs from the latter in the bilobed dor- 



