SILURIAN BRACHIOPODA 43 



pressed. The sinus thus formed bears two strong, rounded plications which 

 are of later origin than the pair which forms its lateral boundaries. The 

 latera bear each 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 (embryonal sinus), thence 

 forward, becoming gradually elevated into a fold which bears three strong 

 rounded plications. Four similar plications are discernible on each of the 

 latera, making in all eleven plications on the entire valve. Umbo incon- 

 spicuous, apex concealed within the foramen of the opposite valve. Con- 

 centric growth-lines obscure, or absent. Average dimensions 12x12 mm. 



These are assumed as the normal characters of adult growth on account 

 of the great predominance of specimens bearing two plications in the ven- 

 tral sinus. 



Variations from the Normal. A. Forms with one plication in the 



ventral sinus. This variation does not attain quite the size of the average 

 normal adult, but retains the same proportion of length and breadth (size 

 10x10 mm.). The surface bears ten plications on the dorsal and nine on the 

 ventral valve. In this form, the embryonal sinus, visible on the earlier por- 

 tion of the dorsal valve, is distinctly continuous with the strong sulcus 

 separating the two plications on the fold in the later and marginal portions 

 of the valve. This variation is not of uncommon occurrence, and immature 

 individuals in various stages of development prove that it is a well-estab- 

 lished genetic difference, and not merely an occasional monstrosity. 



B. Forms with three plications in the ventral simts. The size and pro- 

 portions of the normal are retained in this variety, but the shell bears 

 usually three, sometimes four plications on each of the latera, making ten 

 (twelve) plications for the dorsal, and nine (eleven) for the ventral valve. 

 This form is of comparatively rare occurrence, and is not often noticed in an 

 immature stage of growth. 



