SILURIAN BRACHIOPODA 19 



Dorsal valve flat or slightly concave in the upper part, with the 

 marginal portions abruptly curved upwards in front ; beak small, carrying 

 on its inner side, a large, prominent, triangular 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 produced and abruptly bent 

 downwards, geniculating with the dorsal valve ; beak small, usually per- 

 forated with a small circular foramen ; hinge-line often 50 mm. in length, 

 equaling 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 of the ventral valve, broadly triangu- 

 lar, 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 undulations 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 2, figs, i, la, 1 1) The smallest entire speci- 



men yet detected has a length of 1.25 mm. The outline is semi-oval, with 

 the greatest width near the middle, and about one-fourth greater than the 

 length. Dorsal valve convex in the upper part, becoming concave toward 

 the front. The hinge-area of this valve is very narrow and linear, and 

 carries beneath the beak a small grooved callosity. Ventral valve convex, 

 sloping in all directions from near the foramen, around which the surface 

 is slightly depressed. There is also a depression extending along the middle 

 of the valve to the anterior margin. The place of the beak is occupied by 

 an exsert, conical pedicle-tube, which partly protrudes beyond the cardinal 

 margin of the valve and extends down to, and embraces the dorsal callosity. 

 Cardinal area of the ventral valve comparatively broad, narrowing rapidly 



