DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 347 



regarded as passing the limitations of specific identity ; how- 

 ever, the general form and expression of the shell are charac- 

 teristic, so that, in spite of these variations, no confusion with 

 allied species of the same fauna can arise, nor need there 

 be any hesitation to assign to the different forms a varietal 

 significance only. Probably ten thousand individuals of this 

 species have passed under the observation of the writers, and 

 of this large number fully one-half have been immature forms. 



Specific Characters. 



Normal Mature Form; containing two plications in the 

 sinus of the ventral valve (Plate XVII, figure 21). — Shell 

 sub-triangular or broadly ovate ; length nearly equal to, 

 sometimes slightly exceeding the width. Umbo prominent, 

 sub-acute; cardinal slopes extending one-half the length of 

 the shell, and flattened. 



Ventral valve depressed convex, rounded at the beak ; apex 

 pointed and slightly incurved, exposing beneath it the elon- 

 gate, narrow foramen and the inconspicuous deltidial plates. 

 Dorsum for the first one-third the length of the shell rounded, 

 thence, anteriorly, gradually becoming depressed. The sinus 

 thus formed bears two strong, rounded plications which are 

 of later origin than the pair which forms its lateral boun- 

 daries. The latera each bear three plications with traces of a 

 fourth, making eight (ten) on the entire surface of the valve. 



Dorsal valve somewhat deeper than the ventral, flattened 

 above, depressed near the beak along the median line (em- 

 bryonal sinus), thenceforward becoming gradually elevated 

 into a fold which bears three strong rounded plications. Four 

 similar plications are discernible on each of the latera, mak- 

 ing in all eleven plications on the entire valve. Umbo incon- 

 spicuous, apex concealed within the foramen of the opposite 

 valve. Concentric growth-lines obscure, or absent. Average 

 dimensions 12 x 12 mm. 



These are assumed as the normal characters of adult growth 

 on account of the great predominance of specimens bearing 

 tivo plications in the ventral sinus. 



