258 
PALAEONTOLOGY OF NEW-YORK. 
strongly plicated before reaching half the full size ; and some have plications only 
on the mesial fold and sinus. The plications sometimes bifurcate, as shown in a 
single individual on Plate xlvi and another on Plate xlvii. 
Some specimens lately examined are much more finely and numerously plicated 
than any of those figured : I can regard them, however, only as varieties of the 
same species. 
PLATE XLVI. 
The illustrations upon this Plate show a few of the varieties of form and mark¬ 
ing to which this species is subject. 
Fig. 1 a , b, c. Young shells which are free from plications. 
Fig. Id, e, f. A young specimen which has become very gibbous, and with plications 
strongly developed upon the mesial fold. 
Fig. 1 g, h, i. Three specimens of half-grown individuals, showing the different degrees of 
development in the plications. 
Fig. 1 k, l. A large specimen, in which no plications are developed. The surface presents 
well-marked concentric stripe, and obscure longitudinal stripe on the ventral valve. 
Fig, 1 m. A dorsal valve which is strongly plicate, but shows no mesial sinus. 
Fig. 1 n, o. Specimens showing the plications developed upon the mesial portion of the shell 
in very different degrees of strength. 
Fig. 1 p. Profile of the specimen 1 o. 
Fig. 1 q , r , s. Dorsal, profile and front views of a specimen having plications only on the 
mesial fold and sinus. 
Fig. 1 t , u . Dorsal views of specimens w’ith plications developed over the entire surface of 
the valves, the one with and the other without a mesial sinus. 
Fig. 1 w. An individual showing bifurcating and coalescing striae. 
Fig. 1 x. Profile of the specimen 1 ic. 
Fig. 1 y, z. Front views of specimens, the one with and the other without a mesial sinus. 
PLATE XLVII. 
Fig. 1 a. A specimen having the beak of the dorsal valve broken off, showing the triangular 
cavity beneath the beak of the opposite valve. 
Fig. 1 b, c. Ventral valves of this fossil : the one filled with stone, showing only the tri¬ 
angular cavity A ; and the other having the stone removed by weathering, show¬ 
ing the cavity A and the septum B, presenting the common character of this 
fossil as seen upon the weathered surface of the Lower Pentamerus limestone of 
the Lower Helderberg group. 
Fig. 1 cl. Dorsal valve, showing the entire extent of the cavity formed by the dental lamellae, 
and the central septum below. 
Fig. 1 e. Lateral view where portions of both valves are removed, showing the extent of the 
dental lamellae or spoon-shaped cavity a as shown in fig. 1 d, the central septum 
of the ventral valve B, and one of the septa D of the dorsal valve, which be¬ 
comes free at its extremity. 
Fi<w 1 f. Interior of the hinge ; e,e, the line of junction of the two valves. The septa of 
the upper or dorsal valve correspond to the extension of the dental lamella or 
sides of the spoon-shaped cavity in the lower or ventral valve. 
Fig. 1 g. Dorsal view of a cast, showing the septa D, D ; the cast of the triangular cavity 
beneath the beak of the ventral valve A. 
