DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 319 



projecting over the cardinal area sufficiently to conceal the 

 foramen. Cardinal area broadly triangular, low, incurved; 

 foramen triangular; deltidial plates absent. 



Dorsal valve shallow, nearly flat, slightly rounded over the 

 umbo, but depressed toward the margins. A sharply defined 

 sinus starts near the apex, but by widening with the growth 

 of the shell, it becomes nearly obsolete before reaching the 

 margins. Cardinal line straight; cardinal area narrow, elon- 

 gate 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 

 strise which increase by intercalation; concentric growth-lines 

 rare, except near the margin, where they appear as wrinkles. 



Incipient Form (Plate XV, figures 3, 3 a). — The initial 

 shell of the present 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 greatest 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, accom- 

 panied by one and indications of a second on each of the 

 lateral areas, making five strioe on the valve. On the 

 dorsal valve a low and wide median depression is apparent, 

 bounded by two central strife, these being accompanied by 

 two accessory pairs upon the latera, making six stripe in 

 all. It is very probable that this form represents nearly the 

 actual initial stage in the development of the shell; and if 

 this is the case, the inception of the plications on the sur- 

 face, which become so numerous at maturity (from one 

 hundred to one hundred and thirty on each valve), is syn- 

 chronous with the formation of the rudimentary shell, while 

 in the pauciplicate species here discussed they appear to be 

 of secondary growth. 



