REPORT OF THE STATE PALEONTOLOGIST I9O3 285 



Genus syntrophia Hall and Clarke 

 Syntrophia multicosta sp. nov. 



Plate 5, figures 8-15 



Description. Shell outline semioval, in some specimens inclining 

 toward svibquadrate ; hinge line straight, usually equal to greatest 

 transverse diameter and in a large specimen measuring i6mm. In 

 such a specimen the length would measure 10.5mm and the distance 

 from hinge line to apex of pedicle valve 8mm. Cardinal angles 

 about 90°, not rounded, sides generally rather straight and parallel 

 for a distance reaching nearly to the ends of the transverse axis ; the 

 anterior half of the shell uniformly rounded save for a distinct flatten- 

 ing of the anterior margin. 



Pedicle valve with wide flat cardinal area the sides forming an 

 angle of from 95° to 110° at the beak; beak slightly convex; the 

 slope from beak to valve margin quite straight and nearly uniform 

 in all directions. Delthyritun triangular, two thirds as wide at the 

 hinge line as it is high, and reaching apex. 



Brachial valve nearly flat with a very shallow sinus, not showing 

 in all specimens. 



Radiating costae are numerous and nearly uniform in size from 

 near the point of their origin to their termination on the margin; 

 as shell growth proceeds new costae are added by implantation. 

 Shells about 2.5mm long have some 33 costae, shells of 5mm length 

 have about 49, while adult shells have 81 and over. In figure 13, 

 plate 5, if the two strong costae on either side of the midcosta are 

 traced to their termination on the margin they will be found to have 

 II costae between them instead of one. The new costae do not seem 

 to have been added in regular order, for while the new group of five 

 to the right have their middle one the longest, the middle one of the 

 new five on the left is the shortest and youngest. The costae are 

 crossed by fine raised striae, about .25mm apart. In the gerontic 

 stage the additions to the shell margin of the brachial valve tend to 

 add very markedly to its convexity. 



The interior of the brachial valve shows a strong and prominent 

 median ridge starting from the middle of the valve and widening 



