104 PALEONTOLOGY OF NEW-YORK. 



Fig. 1 i. This shell is a representative of that known as L. ponderosa in Ohio. In form it resembles the 

 two preceding, 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 stris upon both valves resemble the preceding ; but it often 

 happens that those on the convex valve are more or less worn, so as to appear nearly equal. 

 The character of the hinge, with the perforation in the beak, is the same. in both varieties. 

 This variety of form evidently consists of old shells, which often show marks of injuries 

 that have been partially healed ; they also bear evidence of wearing, and are frequently 

 covered with corals, from which the lighter and younger shell is comparatively free. The 

 view of the ventral valve and hinge line is precisely similar to the last; the lower figure is 

 a lateral view, showing the great elevation of the dorsal valve and the abrupt curvature in 

 front, with the strong imbricating lines of growth. 

 Fig. 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 re- 

 presented, and the cardinal line presents the same characters as the previous ones ; the 

 foramen in this one being scarcely closed, while the minute perforation in the beak is very 

 conspicuous. 



This form corresponds precisely with 1 /, while there is a little deviation in the ahernation 

 of the striae in that specimen. 

 Fig. 1 m. The interior of the ventral valve, showing the visceral impression, and the two prominent 

 toothlike processes between which the ligament of attachment passed out through the fora- 

 men : these processes approach each other above, and produce the callosity, which, in most 

 specimens, fills, partially or entirely, the foramen in the opposite valve. The entire inner 

 surface is papillose, and more strongly so on the visceral impression. 



Specimens of this kind, revealing the internal structure of the species, are of rare occur- 

 rence in the Trenton limestone in New-York, though not unfrequently found farther west. 

 Fig. 1 n. The conve.ic valve of this species, with the shell partially exfoliated. The stronger radiating 

 striae 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. 



Although presenting some slight deviations in the striaa from the type of the L. alternata, 

 1 cannot find sufficient to characterize it as a distinct species. 



Plate XXXI. A. 



The elongated forms, figs, l/and k on the last plate, gradually assume a nasute shape, 

 in which state lliey have been characterized as StropJiomena nasuta by Mr. Conrad. The 

 following illustrations will show sufficiently well how the species under consideration 

 assumes this form. 



Fig. 1 ffl. A .specimen which is contracted just below the ears and bent abruptly upwards, having tovrards 

 the front several folds, and a more prominent one directly in front. 



Fig. 1 6. A lateral view of the same specimen, showing the abrupt and extreme deflection of the shell. 



Fig. 1 c. A specimen having the same general form as the last, but contracted and folded in front so as 

 to present precisely the same form as the L. nasuia of Conrad. 



