SILURIAN BRACHIOPODA 27 



Two specimens measure respectively, 19.5 mm. and 14 mm. in length, 

 and 23 mm. and 16 mm. in width, at the hinge. 



Incipient Shell (plate 3, figs, i, la, 3). The form is nearly plano- 



convex. Dorsal va/ve convex on the umbo, flat below. Ventral valve moder- 

 ately convex, with a prominent pointed beak. Hinge narrow, with a small 

 cylindrical perforated pedicle-tube in the center of the ventral area, and a 

 small callosity in the dorsal area. In the smallest specimen observed, the 

 surface is marked by eleven radii on the ventral valve, but is otherwise 

 apparently smooth. Length 2.25 mm.; width in the center, 3 mm. 



DEVELOPMENTAL CHANGES 



On account of the imperfection of the material, it is impossible to trace 

 any minor changes in the outline of the valves, and the specimens indicate 

 that no considerable transformation took place. The modifications in the 

 convexity of the valves is of more importance in this species, and can 

 be readily observed. In the young individuals, up to about one-third full 

 size, the ventral valve is slightly convex and the dorsal valve nearly flat. 

 Further growth of the shell changes these relations, by the gradual deflec- 

 tion of the margin, until the general form of the ventral valve is concave 

 and the dorsal valve is convex. 



The radii appear very early in the growth of the shell, the smallest 

 individual having eleven on the ventral valve, the majority of which extend 

 to the umbo. They probably first appeared in pairs and are found to 

 increase in number afterward by simple intercalation. 



The hinge-area is developed upon both valves in all stages of growth, 

 although in the early stages the cardinal area of the dorsal valve is very 

 narrow, but gradually increases, until at maturity, it is nearly equal to the 

 area of the ventral valve. 



The pedicle-tube is at first cylindrical and short. Advancing in the 

 series, it is found to become conical from growth, and from the widening of 

 the fissure, until, in full grown specimens, it is wider than high. A careful 



