262 SYSTEMATIC PALEONTOLOGY 



Rafinesquina minnesotensis Hall and Clarke, 1892, Pal. New York, vol. viii, 



pt. 1, pi. 31, figs. 25-29. 

 Rafinesquina minnesotensis Winchell and Schuchert, 1893, Geol. Minnesota, 



vol. iii, p. 401, pi. xxxi, figs. 25-29. 



Description. " Shell semi-oblong or semi-oval, with the cardinal angle 

 about 90, or less than 90; diameter from six to nine lines transversely, 

 and from four and a half to eight lines perpendicularly ; the ventral valve 

 convex, sometimes more suddenly deflected after passing the visceral 

 area; dorsal valve gently concave, but reflexed more rapidly about the 

 margin ; the exterior of the convex, ventral valve marked by fine, radiating 

 striae, every third, fourth or fifth one being larger than the intervening 

 ones; interior of the convex (ventral) valve, which is best known from 

 its frequent casts, shows a large muscular impression somewhat bilobate 

 in front and larger in proportion to the size of the valve ; scars of adduc- 

 tor muscles closely approximate, small and in many casts of this valve 

 undistinguishable ; behind they are separated (on the casts) by a short 

 mesial ridge, which between them becomes a narrow mesial furrow and 

 then a deep furrow, terminating at the sinus between the outer larger 

 scars; the outer larger scars (diductors) are radiately striated from the 

 beak (at the base of the dental lamellae small adjusters are occasionally 

 indicated) ; their margins are strongly marked (on the cast) along their 

 posterior sides by distinct grooves formed by the dental plates, which 

 diverge at once from the foramen at an angle of 100-1200, running nearly 

 straight to the outer margins of the muscular scar, when they curve 

 slightly towards the front ; the anterior and lateral margins of the general 

 muscular impression are slightly marked on the casts; outside of the 

 muscular scar is a shallow marginal impressed line which is most evident 

 at the cardinal angles as it converges toward the beak ; the interior edge 

 of the cardinal line is carinate from the teeth to the cardinal angles ; the 

 details of the markings in the apex of the beak are seen on the valve itself 

 to consist of two short, distinct, diverging ridges extending not much 

 beyond the hinge teeth (enclosing the adductor scars), between the 

 anterior ends of which rises a short mesial ridge of about the same size 

 and length, with faint linear ridges parallel with it on each side, which 



