17G PALEONTOLOGY OF NEW YORK. 



Test thin, marked Iiy fine, closely arranged, concentric strise, which are 

 more crowded anteiidily : these are crossed by extremely fine radii, wliich 

 are cliielly confined to the shallow sulcus, anterior to the beak, and are also 

 (jbscurely marked on the wing. 



Ligamental area marked b}- two strong grooves. Two oliliiiiu' linear 

 depressions along the posterior slope indicate the probable existence oi' lat- 

 vy;\\ teetli. Other characters of the interior, unknown. 



The speciiui'ii has a Icuu'tli of I 1 imii.. hci;:iit 7 iinii.. hin^c-linc \'l inui. 



This species bears some resemblance to several species in the Cliemung 

 group, l)ut it is comparatively narrower and more oblique. 



Formation and localiUj. In the Marcellus shale, Bloomfield, Ontario count\-, 

 N. V. 



Lkptodesma Kogersf. 



I'l.ATK XXI, FIGS. l-!l. 

 Leptitilestma Hof/emi. Il.u.i.. Pal. N. V.. vol. v. ].t. 1. Plati-.sMinI t^xiilaii.ntions : PI. ^1, tio-s. l-it. .Ian., 1883. 



Shell ol' small ur medium size, sul)-rhouihoi(hd ; body ovate, very oblique; 

 length greater than the height ; anterior and basal margins broadly rounded ; 

 posterior margin extended and abruptly recurved. 



Valves equally convex above. Right valve somewhat depressed Ijelow, 

 comparatively higher than the left. 



Hinge-line straight, longer than the leiiutli ol' the shell. 



Beaks sub-anterior, obtuse, nearly erect, prominent. Umbonal region 

 gibbous, oblique. 



The anterior extremity is scarcely alate or anrionlnte. consisting of a 

 rdunded extension, straight above and slightly sinuate at the base. Wing 

 comparatively large, triangular, joining the body of the valve near the poste- 

 rior extremity, defined by the crowding and curving of the concentric striae ; 

 margin nearly straight for five-sixths of its extent, then acutely recurving; 

 extremity prolonged into a mucronate spine which extends beyond the poste- 

 rior limit of the valve, in the right valve the wing is less deeply sinuate. 



