No. 109.] 73 



RhYNCHONELLA ACDTIPLICATA. 



Pal. N.Y. Vol. iii, pi. 33, f. 3. 



Shell subquadrangular, compressed : ventral valve depressed- 

 convex, most prominent near the beak : dorsal valve slightly 

 larger than the ventral, most prominent in the middle, declining 

 tvith a gentle curve towards the beak and margins ; beak in- 

 curved. Surface ornamented by about twenty-seven simple, 

 moderately elevated, acutely angular or subcarinato plications 

 on each valve, about five of which are elevated on the dorsal 

 valve into a mesial prominence, which dies out before reaching 

 the beak, and widens rapidly towards the front. On the ventral 

 valve, four of the plications are depressed so as to form a broad 

 rather shallow mesial sinus, with sloping sides, extending about 

 two-thirds of the way to the beak. Shell marked by fine very 

 regular subimbricating concentric lines of growth. 

 A distinguishing feature of this species is the sharply angular or sub- 

 carinate plications : in this character, however, it approaches the last 

 described species ; but its general form is much more compressed, pro- 

 portionally more elongate, with more numerous and finer plications. 



Geological position and locality. Shaly limestone of the Lower Hel- 

 derberg group. 



KhYNCHONELLA ? BIALVEATA. 



Pal. N.Y. Vol. iii, pi. 33, f. 1, 2, 3 & 4. 



Shell small, triangular or triangular-ovate, sometimes compressed : 

 valves nearly equally convex ; beak of dorsal valve incurved ; 

 beak of ventral valve almost straight and subangular ; foramen 

 narrow triangular, and continued to the apex of the beak. 

 Surface ornamented by from twelve to fourteen simple angular 

 plications on each valve ; the two central of which, on the 

 dorsal valve, die out a little before reaching the beak, near 

 which they are somewhat depressed, but towards the front they 

 become slightly elevated above the others, so as to form an in- 

 distinct mesial prominence : the middle plication on the ventral 

 valve is smaller than the others, and depressed near the front 



