258 
PALEONTOLOGY OF NEW-YORK. 
strong concentric undulations, towards the margin abruptly inflected ; ventral valve parallel 
to the dorsal valve, presenting a deep concavity. Surface marked by prominent radiating strise. 
The cardinal area is narrow, and extended to the extremities of the hinge-line ; the foramen 
is broad and spreading, but filled by a callosity of the ventral valve, which has a narrow groove 
at its summit for the protrusion of the pedicle ; the apex of the dorsal valve is often, and per¬ 
haps always, perforated. 
The flatter portions of both valves are strongly marked by concentric undulations, which 
are crossed by finer strise. On the deflected portion there are no undulations, the strise alone 
marking the surface. Sometimes the shell is nearly flat, the deflected portion being either very 
narrow, or not at all conspicuous. The undulations are variable in number, even in shells of 
the same size, and are not to be relied upon as characteristic ; and in very old shells they are 
not so strong as in younger ones, or those of medium size. The strise crossing the undulations 
are likewise variously prominent in different individuals, frequently bifurcating, and in well 
preserved surfaces are crossed by fine concentric strise. The interior structure is always pe¬ 
culiar and sufficiently characteristic, though the exterior characters are very closely simulated 
by a different shell in the Shaly limestone of the Helderberg. 
This species has a wide range, occurring in the Clinton group,, and ranging to the Upper 
Helderberg limestones; and if we include the similar or identical species L. tenuistriata as 
the same, we have the example of a species ranging from Lower Silurian to Devonian, and 
traversing three systems of strata. M. de Verneuil, after examining this species on both sides 
of the Atlantic, and from all the various positions, has concluded that it is identical in all, and 
must be regarded as one species. The specimens from the lower rocks are always smaller, the 
undulations fewer, and the valves less extremely inflected than those of the middle or upper 
silurian, but I have had no opportunity of seeing their internal structure. 
Fig. 6 a, b, c. Dorsal and ventral views of young specimens, where only a narrow portion of 
the margin is deflected. 
Fig. 6 d. A larger individual. 
Fig. 6 e. An individual of the ordinary full grown size, regarded as a mature form. 
Fig. 6 /. A very large individual, having the undulations more numerous, but less conspicuous 
than in fig. 6 e. The extremities of the hinge-line are much extended into acute 
points. 
Fig. 6 g, h. Dorsal view of two small individuals, showing a slight difference in the character 
of the foramen, etc. 
Fig. 6 i. The interior of a ventral valve, showing the muscular impression and dental laminae. 
The specimen is imperfect, but is the only one from this group in which I have 
been able to see the internal structure. 
Fig. 6 k. The interior of the shell, showing the punctate surface. 
Fig. 6 l. The exterior striated surface, with fine concentric striae crossing them. 
Position and locality. In the shale at Lockport, Rochester, Sweden, Wolcott and other 
places : rarely in the limestone of the group. It is extremely common in Europe in the same 
