82 [ Senate 



gins uniting bj §harp prominent interlocking notches : ventral 

 valve flattened or much the less convex, forming a regular el- 

 liptical arch from beak to front, and abruptly deflected upwards 

 at the sides so as to form distinct angles along the lateral mar- 

 gins, the whole front forming a bread truncated projection ; 

 beak somcAvhat obtuse, incurved. Surface marked by strongly 

 elevated, subangular plications, each of which on the front and 

 sides of the sliell has a fine depressed line along the centre, 

 crossed by fine regular concentric zigzag lines of growth. 



This beautiful shell is remarkable for its regular ovoid form, and ver- 

 tically compressed sides. It diflfers from any other species of equal size 

 J^nown to me, by the entire absence of a sinus in either valve. Adult spe- 

 cimens appear to have been generally higher than •wide, though younger 

 individuals ■were doubtless more compressed. It is decidedly the most 

 beautiful RhTjnchonella known to me in all our Atoerican formations. 



etiological position and locality. Oriskany sandstone, Maryland. 



RHT^'CHO^'ELLA BARRANDI. 



Pal. N.y. Vol. iii, pi. 103, f. 3 - 8. 



Shell very large, ovoid or subglobose ; full-grown specimens 

 higher than wide, vertically flattened on the sides : dorsal valve 

 very convex, often extremely elevated ; beak incurved ; cardinal 

 border on each side of the beak profoundly sinuate, fpr the re- 

 ception of the thick, strongly projecting laminse of the opposite 

 valve : ventral valve much the smaller, strongly arcuate lon- 

 gitudinally, having a broad shallow rounded sinus towards the 

 front, abruptly deflected upwards at the lateral margins which 

 are distinctly angular, prolonged in front into a subtriangular 

 vertical projection. Surface marked by forty to forty-six simple 

 (rarely bifurcating) strongly elevated [angular? J plications on 

 each valve. 



This species, the largest of the genus known to me in the rocks of this 

 country, we have yet only met with in the condition of more or less dis- 

 torted internal casts. The large size, however, of the shell, together with 

 its form and the well-defined internal characters usually so strongly im- 



