SILURIAN BRACHIOPODA 15 



apex, but by widening with the growth of the shell, it becomes nearly obso- 

 lete before reaching the margins. Cardinal line straight ; cardinal area narrow, 

 elongate triangular ; beak inconspicuous. Foramen triangular and filled by 

 a tripartite cardinal process which passes into, without filling the foramen 

 of the opposite valve. 



Surface of the shell closely covered by fine thread-like striae which 

 increase by intercalation ; concentric growth-lines rare, except near the 

 margin where they appear as wrinkles. 



Incipient F"orm (plate i, figs. 3, 3^). The initial shell of our series, 



measuring .5 mm. in length and .75 mm. in width, has valves of equal 

 depth and convexity. The length of the hinge-line nearly equals the great- 

 est width of the shell. The cardinal area is high, and equally elevated on 

 each valve. Beaks erect ; foramina large, triangular, open and marginate. 

 On the ventral valve is a single median stria, representing the dorsum of 

 the mature shell, accompanied by one and indications of a second on each 

 of the lateral areas, making three (i. e., five) striae on the valve. On the 

 dorsal valve a low and w-ide median depression is apparent, bounded by two 

 central striae, these being accompanied by two accessory pairs upon the 

 latera, making six stride in all. It is very probable that this form represents 

 the actual initial stage in the development of the shell, and if this is the 

 case, the inception of the plications on the surface, which become so numer- 

 , ous at maturity from (from one hundred to one hundred and thirty on each 

 valve), is synchronous with the formation of the rudimentary shell, while in 

 the pauciplicate species here discussed they appear to be of secondary 

 growth. 



DEVELOPMENTAL CHANGES 



General Form and Outline. In the growth of the shell a change be- 



comes manifest in its outline and relative proportions. The young stages 

 have the width greater than the length, but the more rapid axial growth of 

 the shell reverses these proportions in maturity. Moreover in the incipient 

 stages, the valves, as already noticed, are of nearly equal depth and con- 



