182 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



LINGULA EXILIS (n.s.)- 



Shell broad ovate, moderately convex, length little greater than width; apex 

 obtuse; cardinal margin obtusely rounded; sides regularly curving; base broad- 

 ly rounded Surface lamellose with irregular rug^ or lines of growth. 

 This species is very broad; and the great width at the apex, and broadly rounded 



cardinal extremity, distinguish it among all the other forms of the Hamilton 



group or of the rocks of New- York. 



Geological formation and locality. In the Marcellus shale, near Bridgewater, 



New-York. 



DTSGINA ALLEGIIANIA ( n. s.)- 



Shell broadly elliptical or nearly circular. Dorsal valve depressed conical; anterior 

 side broadly and equally convex; apex placed at a little more than one-third the 

 length from the posterior end of the shell, slightly inclined backwards; the 

 posterior slope concave, and the shell flattened towards the margin. Surface 

 marked by fine regular concentric lamellae, which are distant from each other 

 two or three times their width. 



This large species is one of the finest and most beai^iiful of the genus, nearly 

 equal in size to the Discina grandis of the Oriskany sandstone, and difiering from 

 that one in the posterior position of the apex and the more abrupt sloping on the 

 posterior side, while the concentric strige are much finer and more closely arranged. 

 It differs from the large circular form Discina discus of the Lower Helderberg 

 group, in the greater elevation of the dorsal valve, aud absence of radiating striae. 

 Geological formation and locality. In the Chemung group, Hobbieville, Alle- 

 gany county, New- York. ^ 



Discina alleghania. 



CRANIA HAMILTONI^ ( n.s.). 



Shell subconical, subcircular; apex subcentral, pointed in well-preserved speci- 

 mens, often worn or decorticated. Exterior surface of the dorsal or upper valve 

 lamellose. Ventral or lower valve marked by four strong muscular impressions, 

 which are variable in form : the two lateral ones are distant, and each apparently 

 double; the central impressions approximate, diverging above and assuming a 

 somewhat cordiform appearance; vascular impressions strongly digitate. 

 This species is found adhering to valves of Tropidoleptus, Strophodonta, Spirifer, 



Spirigera, ^vicula, Orthoceras; and the separated valves are free in the shales. 

 Geological formation and localities. In the shales of the Hamilton group : 



Western New-York, Maryland, and Virginia. 



