»«8 . PALEONTOLOGY OF NEW-YORK. 



strongly plicated before reaching half the full size ; and some have plications onlj 

 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. 1 d, 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 ^, /. A large specimen, in which no plications are developed. The surface presents 

 well-marked concentric striae, and obscure longitudinal striae on the ventral valve. 

 Fig, 1 m. A dorsal valve which is strongly plicate, but shows no mesial sinus. 



Fig. 1 n, 0. 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 with plications developed over the entire surface of 

 the valves, the one with and the other without a mesial sinus. 



Fig. 1 to. An individual showing bifurcating and coalescing striae. 



Fig. 1 X. Profile of the specimen 1 w. 



Fig. 1 y, 2. Front views of specimens, the one with and the other without a mesial sinus. 



PLATE XLVII. 



Figi 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 5, 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. 



Figi 1 d. Dorsal valve, showing the entire extent of the cavity formed by the dental lamellis, 

 and the central septum below. 



Flg> 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. 



Fig. 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. 



