466 CRETACEOUS PALEONTOLOGY. 



Trigonia kummeli n. sp. 

 Plate XLVIIL, Figs. 11-12. 



. Description. Shell small, the dimensions of an average speci- 

 men being : length about 26 mm. ; height, 20 mm. ; convexity,. 

 6 mm. Ovate-subtrigonal in outline, moderately convex in front, 

 the greatest convexity being near the anterior margin, compressed 

 posteriorly. Beaks rather sharp, nearly anterior, slightly re- 

 curved. Anterior and antero-basal margin describing nearly a 

 semicircle; postero-basal margin straighter and sloping upward 

 towards the posterior hinge extremity; dorsal margin gently 

 concave. Surface of the valves divided into two regions by a 

 ridge, or more properly by a row of nodes passing in a concave 

 line from the posterior side of the beak to the posterior margin 

 a little below the hinge extremity. The lower portion of the 

 valve marked by 14 or 15 sharply angular, prominent, narrow,, 

 nodose ribs, the most anterior ones of which curve strongly for- 

 ward in passing from the dorsal extremity to the shell margin;, 

 the first two or three ribs on the beak are crowded close together, 

 the interspaces gradually becoming broader to about the sixth- 

 rib, beyond which the interspaces are about equal in width, being 

 much wider than the ribs themselves. The upper surface of the 

 valve continues in the general slope of the valve from the row 

 of bounding nodes to over half the distance to the hinge-line, 

 the surface is then sharply inflected for a short distance and then 



again deflected, when it continues to the hinge margin in nearly 

 the plane of the valve, this deflected portion of the two valves- 

 forming a keel-like projection of the shell along the hinge-line 

 back of the beaks. The ribs of the lower portion of the shell are 

 bent abruptly forward as they cross the bounding line between 

 the two portions of the valve; they continue in that direction to 

 about the middle of the broad, inferior part of the upper portion, 

 when they are bent abruptly backward to the lower margin of 

 the inflected portion, where they are again bent forward to the 

 hinge margin ; towards the posterior extremity of the shell tha 

 ribs become more or less indistinct. In addition to the ribs the 

 shell is marked by concentric lines of growth, which are indistinct 

 except in front near the margin. 



