No. 89.] 



77 



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 rugae 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. 



DISCINA ALLEGHANIA (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 beautiful of the genus, nearly 

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

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

 posterior side, while the concentric striae 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, and absence of radiating striae. 

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

 gany county, N.York. 



DiSCIJfA ALLEGHANIA.. 



CRANIA HAMILTONIiECn.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 apparent- 

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

 a somewhat cordiforra appearance; vascular impressions strongly digitate. 

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



Spirigcra, j^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. 



