54 MEMOIRS OF THE STATE MUSEUM 



Beak. In the first stage, the ventral beak is high and slightly re- 

 supinate, exposing the foramen in an inclined plane. It gradually shortens 

 and becomes erect, and when the shell attains a length of 8 mm., it is bent 

 forward, the cardinal area being slightly incurved. Thereafter, the inflec- 

 tion of the area increases, concealing first the deltidial plates, and finally 

 the foramen, until, in maturity, the beak lies appressed upon the embryonal 

 sinus of the dorsal valve. 



Foramen. In the initial shell, this is undoubtedly triangular and 



free from deltidial plates. With the starting point of our series, however, 

 plates have begun to develop, thus narrowing the pedicle-aperture, and 

 rounding its basal angles. With the growth of the plates more rapidly 

 along the lower portion of their inner edges, the foramen shortens quickly, 

 while narrowing but slowly, assuming in the second stage (fig. 16), a 

 lanceolate, in the third stage (fig. 17), an oval, and in the fourth stage (fig. 

 18), a broadly circular outline. In the last two of these stages, the deltidial 

 plates have come in contact with each other above the apex of the dorsal 

 valve, and the pedicle-aperture itself has, from the second, if not from the 

 first stage in the series, encroached upon the apex of the valve, so that, as 

 it attains a circular outline, one-half its periphery is formed by the substance 

 of the valve itself, and the other half by the deltidial plates. From this 

 stage upward, there is no apparent change in the actual dimensions of the 

 foramen, and, therefore, with the growth of the shell it becomes relatively 

 much smaller. It appears, however, that with the incurving of the cardinal 

 area and the concealment of the deltidial plates, the foramen becomes more 

 and more inclosed by the apical portion of the valve, and it may be that 

 actual contact with the deltidial plates in the last stage of development is 

 lost. In this final stadium, with the procumbent position of the ventral 

 beak upon the dorsal valve, the plane of the foramen is parallel to the sur- 

 face of the dorsal valve, and the aperture is therefore lost to sight, or visi- 

 ble only at its upper edge. 



