23O NEW YORK STATE MUSEUM 



interspaces. A comparison of these forms with larger collections, showing 

 numerous intermediate phases, makes it impossible to draw lines of specific 

 distinction, and it is doubtful if any fixed varietal form exists." 



Notwithstanding this expression, my observation leads to the convic- 

 tion that this narrow acuminate form of the shell is frequent and persistent 

 in the abundant collections before me. That it is not a casual expression 

 is evinced by a number of specimens which are rotund and retain their 

 normal convexity. The coarser or less abundant plication of the surface 

 originally given as a character of L. acutirostrum, is not always 

 persistent, yet in the best preserved specimens the difference in this respect 

 is noteworthy. 



Diagnosis. Shells acuminate, apical angle as measured between the 

 sical edge and the ridge of the posterior slope, 6o°. Lateral margins 

 long ; the hiatus extends for nearly two thirds the length of the valve, its 

 apparent length being often increased by compression ; straight or with 

 a gentle inward curvature. Sicae moderately broad near the apex, rapidly 

 diminishing in width. From the very short, triangular cardinal area the 

 posterior margin slopes downward with a very gentle curve. This margin 

 lies almost vertically beneath the umbonal slope of the valves so that 

 under compression it is concealed and the straight umbonal slope makes 

 the apparent posterior margin. 



Surface convex. The apex is somewhat incurved, and the convexity 

 of the valves is greatest near the sical margin. Thence the slope to this 

 margin is somewhat abrupt, but toward the posterior margin more gradual 

 to near the edge, whence it becomes almost vertical. Over the pallial 

 region the convexity is much more regular. 



The surface markings consist of continuous, simple, radial plications 

 separated by relatively narrow interspaces. These plications vary in size 

 on different parts of the valve ; they are largest where shortest, on the 

 vertical slope to the posterior margin close to the umbones, where they 

 strongly crenulate the margin and form an accessory to articulation. From 

 this margin they decrease in size posteriorly, and near ihe hiatus there is an 

 area which is free of striation. When normally preserved, these ribs are 



