274 



PALJEONTOLOGY OF NEW-YORK. 



This is a unique little species, appearing, in the young state, in very rotund forms, with a 

 pointed elevated beak, beneath which there appears a minute oval or triangular foramen, which 

 undergoes some change with the advancing growth of the shell. The number of plications is 

 usually about five or six on each side of the smaller ones in the centre ; and there is scarcely 

 any increase of number in older shells, but they become broader, so that the appearance of the 

 surface is variable from this cause. The beak is more prominent in young specimens than in 

 older ones, where it is often scarcely elevated above the hinge-line. Although variable in form 

 and proportions, it is readily distinguished from other species of the group. 



Fis^. 3 a, h. Ventral and profile views of a young specimen. 



Fig. 3 c, d. Ventral and profile views of a larger specimen than the preceding. 



Fig. 3 e. Ventral view of a specimen having the beak of the dorsal valve well preserved. 



Fig. 3 /. The beak of the same enlarged, showing the foramen. 



Fig. 3 g. Profile view of the same. 



Fig. 3 h. Dorsal view of a specimen, larger than the preceding. 



Fig. 3 7, k. Dorsal and ventral valves of a larger compressed specimen. 



Fig. 3 I, m. Views of a more rotund specimen, showing the beak much reduced, with the fora- 

 men not surrounded. 



Fig. 3 7z, 0. Specimens which are more extended laterally than the preceding forms, the last one 

 being almost cylindrical. 



Fig. 3 p, r. Front views of two specimens, showing slight indications of a sinus, which, however, 

 never becomes an important feature. 



Fig. 3 t. The beak and foramen of a full-grown individual. 



Position and locality. In the shale of the group at Lockport. 



456. 27. ATRYPA NEGLECTA. 



PL. L VII. Fiff. Iffl-p. 

 Atrypa neglecta, page 70, pi. 23, fig. 4 a-f, this volume. 



Ovoid or subpyramidal ; beaks acute ; shell gradually enlarging from the beaks to the base, 

 which, in old shells, is deeply sinuate ; ventral valve more convex than the dorsal valve ; 

 surface marked by simple sharp plications, which are crossed by fine concentric striae, and 

 sometimes by a few concentric imbricating lines of growth ; dorsal valve with a mesial sinus 

 below the middle, and a corresponding fold upon the ventral valve. 



In the young shells the valves are equal, and there is neither sinus nor elevation ; but as the 

 shell advances in size, the sinus becomes conspicuous. There are generally three and sometimes 

 four plications in the sinus, and four or five elevated on the opposite valve. The plications 

 usually appear as if smooth, except near the base, where there are some strong imbricating 

 lines of growth. It is a very common species, and sufficiently distinct in all its phases to be 

 readily recognized. 



