DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 323 



offer a series of changes, not often traceable in Silurian 

 brachiopods. 



The development of the characters of the hinge-area is also 

 very satisfactorily demonstrated, and affords some interesting 

 points of comparison with certain forms of Orthothetes and 

 Strophonella. These features are noticed at the end of the 

 description of the species Strophonella striata. 



Specific Characters. 



Mature Form (Plate XVI, figures 4, 4a, 10, 13). —Shell 

 semi-elliptical or semi-circular in outline. 



Dorsal valve flat or slightly concave in the upper part, 

 with the marginal jjortions abruptly curved upward in front; 

 beak small, carrying on its inner side a large, prominent, tri- 

 angular callosity, grooved along its summit and nearly filling 

 the area of the opposite valve. 



Ventral valve usually convex in the upper part, becoming 

 flat or concave below, and with the marginal portion pro- 

 duced and abruptly bent downward, geniculating with the 

 dorsal valve; beak small, usually perforated with a small 

 circular foramen; hinge-line often 50 mm. in length, equal- 

 ing or greater than the width of the shell below; cardinal 

 extremities twisted and often much extended ; cardinal area 

 narrow, edges parallel, formed by both valves ; deltidial area 

 broadly triangular, occupied by the grooved callosity under 

 the dorsal beak. 



Surface marked by regular, rounded, radiating striae. 

 From the beaks to the curtain, or geniculated portion, the 

 shell is ornamented with regular, strong, concentric undula- 

 tions or corrugations. 



This species varies greatly in size and form, in the different 

 horizons and localities where it is found. In many places 

 the mature shells are about half the size of the specimens 

 from Waldron. 



Incipient Form (Plate XVI, figures 1, 1 a, 11). — The small- 

 est entire specimen yet detected has a length of 1.25 mm. 

 The outline is semi-oval, with the greatest width near the 



