PALAEONTOLOGY 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. 
Sukface marked by strong concentric lamellose striae, and finer closely 
arranged radiating striae. Shell, when exfoliated, showing radiating 
striae. 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 striae, while the concentric lines are still preserved ; 
and a yet further exfoliation reveals another set of radiating striae, 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 regular and rectilinear in their arrange¬ 
ment. 
This species bears some resemblance to D. forbesi; 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 showing radiating striae. 
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 he 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 mountain near Hudson. 
