26 PALJEONTOLOGY OF NEW YORK. 



deltidimn, with diverging deltidial ridges, outside of which are the areal bor- 

 ders, all marked with successive transverse growth-lines. This sheath appears 

 to have been entirely enclosed by the shell about the umbo, but just at this 

 point all the specimens before us are a little broken, and it is difficult to deter- 

 mine whether or not its apex was extruded for a short distance. The slight 

 gaping of the valves on the cardinal margin indicates that this was probable. 



Beneath the shelf thus formed lie the umbonal portions of the progressive 

 muscular scars. The central and lateral scars are strongly developed, 

 passing forward from the pedicle-shelf in three broad ridges, which 

 terminate at the anterior margin of the platform. The crescentic mus- 

 cular fulcrum is but faintly visible on this valve, though a broad flabellate 

 scar, apparent on each side, probably comprises the terminal and posterior 

 parietal impressions. The platform is comparatively thin and is solid, showing 

 but slight excavation on its anterior wall. From its center a low median ridge 

 extends about half-way over the anterior area of the valve ; its margins are 

 excavated by shallow furrows, which unite at the extremity of the ridge and 

 extend to the anterior margin of the valve ; on either side of this median fur- 

 row is a broad, low elevation, the entire arrangement being equivalent to that 

 seen in the pedicle-valve of Lingvlops Whitfieldi, though the features are not so 

 conspicuously developed. 



In the opposite or l)racliial valve the crescentic ridge is very strongly 

 developed, its central portion or crown being separate from the lateral 

 portions. From its posterior angle the sides diverge for a short distance, 

 ending- abruptly ; and from outside and behind these short arms, the ridge 

 again appears, being broader and flatter than before, and sweeping in broad 

 sub-marginal curves to the anterior edge of the platform. The external scars 

 are more distinctly seen than in the pedicle-valve, but they are not very satis- 

 factorily defined. From the crown of the crescent, a low axial septum passes 

 forward to the edge of the platform, dividing the central scar, on either side 

 of which may be seen the lateral impressions. The platform is moderately 

 high and its edge is a little incurved between each ante-lateral angle and the 

 median septum. The latter feature is very strongly developed where it joins 



