SILURIAN BRACHIOPODA 33 



character which constitutes the most conspicuous change in the shell in 

 its growth from the young to the fully mature condition. 



SPECIFIC CHARACTERS 



Mature Form (plate 3, figs. 15, 16, 16a). Shell ventricose. Out- 



line transversely subelliptical, sometimes nearly as long as wide. 



Ventral valve convex, depressed in front, forming a more or less 

 defined sinus which carries four or five of the plications ; beak short, acute ; 

 area short, broadly triangular, usuallj^not exposed. 



Dorsal valve gibbous, with the central portion elevated, frequently 

 presenting a broad undefined median fold ; beak incurved under the beak of 

 the opposite valve ; umbo prominent. . 



Surface marked by about fifteen strong, simple, elevated, rounded or 

 angular plications on the body of the shell, and smaller bifurcating plica- 

 tions on the latera. Occasionally intercalated plications are present on the 

 middle of the valves. The plications are crossed by fine arching striae of 

 growth, which are sometimes aggregated, forming conspicuous concentric 

 lines or varices of growth. 



Mature shells measure from 11 to 17 mm. in length, and from 12 to 19 

 mm. in width. The depth of the conjoined valves varies from 9 to 12 mm. 



Incipient Form (plate 3, figs. 14, 14a). The smallest shell observed 



has a length of 2 mm. and a width of 2.25 mm. The dorsal valve is slightly 

 more convex than the ventral and is a little shorter. Eight rounded plica- 

 tions are shown, five of which extend to the umbo of the valve. A short 

 plication is intercalated in the middle and there is also a short one on each 

 side of the valve. Ventral beak small and elevated, with a broad, triangular, 

 open area below. 



DEVELOPMENTAL CHANGES 



The series of specimens selected to represent the development of this 

 species contains fifteen normal individuals, varying from a length of 2 mm. 

 to a length of 1 7 mm. The proportions of length and width remain nearly 

 constant throughout, the width being somewhat the greater. 

 5 



