160 PALiEONTOLOGY OF NEW-YORK. 



flat : apex rising a little in front of the foramen, and concave between 

 this point and the anterior margin. Depression of the surface at the 

 foramen round or round-oval ; the slit oval or acute at the extremity. 

 Surface marked by strong concentric lamellose stria?, and finer closely 

 arranged radiating stria3. Shell, when exfoliated, showing radiating 

 stria). Interior of shell with distant strong radiating striae, simple or 

 fasciculate, and vascular impressions strongly marked. 



The specimens of this fossil are usually quite orbicular, and the deviations from 

 that form may be due to distortion. When not exfoliated, the surface is marked by 

 strong lamellose nearly equidistant concentric striae, and very fine closely arranged 

 radiating striae. Few specimens, however, preserve these characters in perfection. 

 ' Exfoliation destroys the finer strise, while the concentric lines are still preserved ; 

 and a yet further exfoliation reveals another set of radiating strise, which are more 

 or less connected with the vascular impressions. In fig. 14 of the plate, these ra- 

 diating striae are represented as much too reguljir and rectilinear in their arrange- 

 ment. 



This species bears some resemblance to D. forlesi ; but the lamellose striae are 

 finer, and the foramen is longer and narrower, according to the figures in the Memoirs 

 of the Geological Survey of Great Britain, while the figures of Mr. Davidson re- 

 present the apex of the lower valve as much more excentric than in our specimens. 



Fig. 13. The lower valve partially exfoliated, and sliowing radiating striaa. 



Fig. 14. Interior of the lower or ventral valve, showing the lamellose structure and radiating 



striae. [In the figure the striae are too regular, and should be represented as 



bifurcating towards the margin.] 

 Fig. 15. A lower valve (with the margins slightly exfoliated), preserving nearly entire the 



concentric lamellose striae. 

 Fig. 16 a ( Error for 15 a). Profile of the lower valve, showing the convexity. 

 Fig. 15 b. Enlargement of the lamellose striae. 

 Fig. 15 a ( Error for 14 a). Enlargement of the radiating striae of fig. 14. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Becraft's mounbiin near Hudson. 



