5 8 MEMOIRS OF THE STATE MUSEUM 



.8 mm. in width, and at this stage of growth, the incipient shell has mani- 

 festly not received much increment. That this, however, is not the actual 

 primitive shell seems proven by indications of two very indistinct con- 

 centric growth-lines, and by the presence of faint radiating plications near 

 the anterior margin, between the second growth-line and the margin itself. 

 It is very probable that the incipient shell consisted of that portion of the 

 individual (plate 5, fig. i) lying within the first growth-line, and as 

 this would make its size about -5X-4 mm., this fact in itself is sufficient 

 apology for our not having detected the earliest stage of its development, 

 even if other causes had permitted its preservation. 



Beaks. In all normally developed individuals less than 5 mm. in 

 length, the beak of the ventral valve is erect and exsert. At about this 



o 



stage of growth, a tendency to apical incurvature is manifested, which 

 increases up to maturity, when, under normal development, the entire 

 umbo is evenly incurved, concealing the deltidial plates and often much of 

 the foramen. On the dorsal valve, the beak is quite obscure in the young- 

 est forms, and in later stages of growth is concealed beneath the deltidium 

 or incurved beak of the opposite valve. 



Foramen. This appears first as a simple triangular opening, its 

 apex reaching to, but not truncating the apex of the umbo, and it is re- 

 tained in this condition until the shell attains a length of at least 3 mm. 

 of normal growth. At this age, the deltidial plates begin to form, making 

 their first appearance as two minute triangular laminae, taking their origin 

 in the basal angles of the foraminal triangle, and giving the foramen a 

 lanceolate outline. 



By increments to their internal edges, these plates presently come in 

 contact with each other, truncating the interior basal angle of each, the 

 plates being, from this period onward, in progressive symphysis. The incre- 

 ments to these plates are made more rapidly at and about their interior 

 angles, and as a result, the foramen assumes successively an elliptical, an 



