68 



broken examples, and the descriptions of the species are very short and 

 indefinite. 



The posterior pointed extremity of the English sj)ecimen8 are broken 

 oif, just as they are in the shells described above. In North America, 

 as most collectors are aware, the pointed ends of the valves of living 

 Unionidoe are often bitten off bj- muskrats and other animals, whose 

 instinct teaches them which is the most fragile part of the shell and how 

 best to get at the (to them) luscious morsel inside. In the case of the 

 fossils, however, the fracture is probably accidental. 



Trigonia diversicostata. (N. Sp.) 

 Plate X., figure 1. 



Shell moderately inflated, elongated, scaphoid or subarcuate, very 

 inequilateral ; anterior end short and rounded ; posterior side produced, 

 cuspidate and bluntly pointed above. Beaks large, projecting, recurved, 

 anterior, nearly terminal. Lunule none; posterior area (of the two 

 united valves) broadly ovate lanceolate, with a rounded margin. Hinge 

 boi'der, behind the beaks, straight or slightly concave, sometimes with a 

 very gentle downward inclination, which becomes a little more decided 

 at the tip of the beaked posterior extremity. Yentral margin broadly 

 rounded, but the upward curve is always greatest behind. The short 

 anterior end is always much wider than the posterior extremity, and the 

 margin of the latter is convex below and almost straight above. As 

 measured in the centre, behind the beaks, the height is less than half the 

 length : the thickness or convexity is nearly equal to the height. 



Surface boldly ribbed, with a very singular and complex styl^e of cos- 

 tation. At the anterior end the general direction of the ribs is almost 

 horizontal, but near the margin they are cmwed, and ultimately straighten 

 and trend upwards. In the basal half of the shell, however, they are either 

 undulated or broken up into a series of zigzags. Thirteen or foui-teen of 

 these horizontal ribs commenced at the margin of the anterior end; then, 

 neai' the middle of the valves, five or six of them suddenly bend upwards 

 at a sharp angle, and become either transverse, or at length incline a 

 little forwards. About one-half of the ribs which proceed from the 

 anterior end are not continuous, but are distinctly truncated by the up- 

 ward bend of those which reach to the superior border. This is most 

 obvious in the umbonal region, for on the beaks the first five anterior 

 ribs are cut oif, as it were, by the upward bend of the sixth. Below 

 this the anterior costae seem alternately continuous and interrupted. 



