258 



PALilONTOLOGY 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 striae. 



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 pcilicle ; the apex of the dorsal valre is often, and per- 

 haps always, perforated. 



The flatter portions of both valves are strongly marked by concentiic undulations, which 

 are crossed by finer stria?. On the deflected portion there are no undulations, the stria; 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 >!> very old shells they are 

 not so strong as in younger ones, or those of medium size. The striee crossing the undulations 

 are like^vise variously prominent in dilferent individuals, frequently bifurcating, and in well 

 preserved surfaces are crossed by fine concentric stria;. The interior structure is always pe- 

 culiar and sufliciently 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, h, c. Dorsal and ventral views of young specimens, where only a narrow portion of 



the margin is deflected. 

 Fig. Q d. k 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 laminss. 

 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 I. The exterior striated surface, with fine concentric stria; 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 



