PLATE 31. 



Fig. 1. 141. 4. LepTwENA altebnata. (^'og- 102.) 



1 a. A (pecimcn of mediuin size, with the elevated striiD very distinctly marked, and the gradual enlarging of 

 nne of the intermediate smaller striic is shown as they recede from the apex. 



I b. A smaller specimen, presenting the characters of the species in a very perfect manner. 



1 4*. Enlarged portion of the surface. 



1 e. Inside of the dorsal valve of this species, showing imperfectly the visceral impression near the beak, and 

 the papillose surface of the shell. The alternation of larger strisB with fascicles of smaller ones is quite 

 distinct. 



1 d. An individual of larger size, having the essential features of the species well preserved. This specimen ia 

 much more convex in the centre, and more abruptly curved in front, than either of the preceding. 



1 e. Dorsal valve of another specimen, in which the length and breadth of the shell are about equal; and in 

 this respect only does it differ from the preceding specimens, in which the width considerably exceeds 

 the height The larger and more elevated striae alternate precisely in the same manner with four to six 

 smaller ones, the whole being crossed by fine concentric slightly undulating lines. 



1/'. A specimen having almost precisely the same form as 1 e ; but the larger stris are not so prominent, and 

 the intermediate smaller ones not as regular as in the last specimen. 



1 g. The dorsal valve of this species, from the shaly blue limestone of Ohio. The surface is well preserved, 

 and the striae very strongly marked : a few imbricating lilies of growth are visible near where the cur- 

 vature of the shell becomes abrupt. 



1 h. Ventral valve of another specimen : the radiating strite are nearly equal ; the larger elevated ones, dividing 

 the fascicles of small rays, being scarcely distinct, while on the dorsal valve they present the same 

 distinction as in the last figure. The hinge view shows the deltoid foramen nearly closed, with the small 

 perforation in the beak, which is enlarged in the upper figure. 



1 t. This shell is a representative of that known as L. ponderoaa in Ohio. In form it resembles the two pre- 

 ceding, except that it is more deflected on the margin, with more distinct imbricating and squamose 

 lines of growth ; the space between the two valves is also much greater, and the shell more massive. 

 The lower figure is of the dorsal valve, the two upper representing cardinal and lateral views. 



1 t*. Magnified view of a coral upon the shell. 



1 k. An elongated form of this species ; the dorsal valve showing the margin abruptly curved upwards from a 

 little below the hinge line. The alternating striae are very perfectly represented ; and the foramen is 

 scarcely closed, while the minute perforation in the beak ia very conspicuous. 



1 m. Interior of the dorsal valve, showing the visceral impression. 



1 m*. Enlarged portion of a coral which partially covers the interior of the shell, while the upper and left- 

 hand margin is covered with ChceUtea lycoperdon. 



1 n. The convex valve of this species, with the shell partially exfoliated. The stronger radiating strie are very 

 prominent, but the intermediate ones are obsolete except towards the hinge ; the concentric elevated 

 lines are, at the same time, very prominent. The shell is partially removed, so as to reveal somewhat of 

 the internal structure. 



