TIIK I'AUOONTOLOUY ( >K M I N N KSOTA. 



[Orlhls i I ):i lni:i tit Mil I h;i in hu T'^C||M|S . 



OKTIMS ( DAI, MAN KM, A) n AMIUHKIKNSIS? M'nlrolf. 

 PLATE \\\in, i u.s n if, 



1SS|. (h-l/iis lniiiiliiii-1/.'iiHin \V.\l.roTT. Monograph <il' (.lie I!. S. (icnl(ij.vic;il Survi-.v. vol. viii, p. 7:1, 



pi. M, llR. f.. 



ititni: "Shell small, siihorbicular in outline, plano-convex; hinge- 

 lino n little slim -ter than the greatest breadth of the valves. Dorsal [ventral] valve 

 moderately convex, most elevated a. lit I In behind the center, along a slight ridge 

 formed by two or three strong, slightly raised stria-; beak small, depressed about 

 halfway down the cardinal margin. Ventral [dorsal | valve depressed, the slightly 

 convex, mesial depression well detined IVom the beak to the front margin. 



"Surface of bolli valves marked by from twelve to eighteen strong angular Striee, 

 which increase by bifurcation or inl.erca.la.tion towards t.he nia.rgin." 



The following description is based on .Minnesota, material: Shell very small, 

 snhqiiadrale in outline, hinge-lino equal to, or slightly less than, the greatest, width 

 below; cardinal angles rectangular; sides in the posterior third straight or gently 



convex; anterior angles and front, broadly rounded. Ventral valve convex, subcar- 







inato along the middle, with a ll.it slope toward the sides; greatest elevation about 

 one-third the length of the shell from the posterior margin. Cardinal area wide, 

 broadly triangular, slightly concave, forming an angle of about 115" with the plane 

 of the lateral margin; delthyriiim narrowly triangular, more than twice as long as 

 wide, with a linear elevation along each wall, lieak slightly incurved, raised above 

 that of the dorsal v.ilve. Dorsal valve less convex than the ventral, with a sinus 

 beginning immediatly below the beak and rapidly expanding into a broad, shallow 

 depression, which produces a more or less undulated anterior margin. Cardinal 

 area conspicuous, slightly concave, less than half as wide and more erect than that 

 of the other valve; dellhyrinm about as long as wide, bounded on each side by a 

 linear elevation, and occupied centrally by a narrow and simple cardinal process. 



Surface with moderately strong, radiating, bifurcating, angular stria-, of which 

 from thirty-two to forty-six may be collided along the anterior margin. In some 

 specimens the stria- are nearly e.|iial in si/e, while in others those originating on 

 the umbones increase rapidly in strength, and the ones coming in later by bifur- 

 cation remain smaller, giving to sueh shells the appearance of stria- in bundles. 

 Interior features unknown, except that the dental plates are strong and attached 

 to the bottom of the valve. 



Since the Cardinal areas, delthyrium and interior characters are unknown in the 

 Nevada specimens, the writers are not satislied that the Trenton forms are identical 

 with (). liiiHihuriirnxix. A specimen of the latter was sent to Mr. Walcott, who writes 

 that "(he cardinal line is a trille. shorter in (.he Nevada specimens than in those from 



