DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 373 



viduals having a length of 2.5 mm. and a width of 1.75 mm., 

 the adult form measuring 9 mm. in length by 7.5 in width. 



In all stages of growth earlier than that approximately 

 indicated by a size of 6 X 4 mm., it is very difficult, and from 

 the present observations impossible, to draw the line of separa- 

 tion between this species and W. nitida, and the fact which 

 has been demonstrated for Dalmanella elegantula and Rhipid- 

 omella hybrida ; namely, that in the earliest growth-stages no 

 specific differences are manifest, will be probably found to 

 hold good for these two species also. And in the latter case 

 a considerably larger size is attained by the embryonic forms 

 than is reached by the former species, before the differential 

 characters are assumed. This is due to the fact that these 

 two species, when mature, have essentially no surface sculp- 

 ture, and differ less in general form and outline than do the 

 mature individuals of Dalmanella and Rhipidomella. 



Specific Characters. 



Mature Form (Plate XXI, figures 12, 12 a, 12 5). Shell 

 sub-pentagonal or ovate; beak erect, acute, and prominent, 

 rapidly widening toward the base. Lateral margins nearly 

 straight for about one-third the length of the shell, thence 

 rounding to the anterior margin. Valves about equally con- 

 vex, giving the shell a sub-lenticular contour. 



Ventral valve with attenuate, straight, or slightly arcuate 

 beak. Foramen triangular and without deltidial plates. 



Dorsal valve more nearly sub-pentagonal in outline ; beak 

 incurved into the foramen of the ventral valve. 



Surface smooth, or in rare instances showing a faint 

 pseudo-punctate appearance which is entirely superficial. 

 Dimensions of average adult 9 X 7.5 mm. 



Incipient Form (Plate XXI, figures 4, 4 a). Shell measur- 

 ing 2.5 x 1.75 mm. Oval, proportionally longer and nar- 

 rower than in the adult state. Beak elevated, acute, straight. 

 Foramen of the ventral valve very broad, triangular, extending 

 to the apex. Dorsal beak full, rounded, and inconspicuous. 



