DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 389 



tral sinus have them developed early in the growth of the 

 shell, and they usually increase regularly to the time when 

 the full size of the shell is attained. 



Beaks. — The beak of the ventral valve in its earliest con- 

 dition is commonly erect, pointed outward, and of a broad 

 triangular form, while that of the dorsal valve is small, not 

 prominent, and lies in the longitudinal axis of the shell. 

 In all cases the subsequent deepening of the valves tends to 

 incurve the beaks toward the cardinal area. The degree of 

 incurvature varies greatly in the different species. Meristina 

 rectirostris shows a minimum, and Meristina Maria or Atrypa 

 reticularis a maximum, and between these limits all inter- 

 mediate conditions occur. The usual degree of incurvature 

 is presented in Spirifer radiatus, Homoeospira evax, and the 

 Rhynchonellidse. 



The outlines on Plate XVIII, illustrating the profiles of 

 the beaks in a series of Rhyncliotreta cuneata, represent an 

 uncommon condition, for in this species the ventral beak, 

 from its divergent initial position, gradually approaches, and 

 at maturity attains parallelism with the longitudinal axis of 

 the shell. It never becomes sufficiently incurved to conceal 

 to the slightest degree the deltidial area, while the initial 

 dorsal beak becomes more and more incurved, until, finally, 

 it lies entirely within the ventral umbonal cavity. 



Those species furnished with a circular apical perforation, 

 as Atrypa reticularis, Homoeospira evax, and Rhynchotreta 

 cuneata, lose the initial point of the ventral beak from absorp- 

 tion, due to the increase in the size of the perforation or to 

 its final terminal position. In Atrypa reticularis, or Meris- 

 tina Maria even, both beak and perforation are destroyed, 

 from the forcing of the ventral beak into contact with the 

 dorsal umbo, produced by the great increase in the depth of 

 the valves from growth along their anterior margins. 



Cardinal Area. — Omitting for the present the Stropho- 

 menidas and Orthidce, the initial state of the ventral cardinal 

 area for all other forms is a broad triangular opening beneath 

 the beak, with simple sharp margins. This condition is 



