SILURIAN BRACHIOPODA 69 



Incipient Form (plate 7, figs. 4, 4^'). Shell measuring 2.5x1.75 



mm. Oval, proportionally longer and narrower 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 incon- 

 spicuous. Shell convex just below the beak, becoming depressed toward 

 the anterior margin. 



DEVELOPMENTAL VARIATIONS 



General Form and Outline. In the incipient stadia of growth, the 



shell is extremely elongate and quite perfectly oval ; the beak of the ventral 

 valve is relatively broad, its lateral margins having a slight outward curve. 

 With growth, the shell broadens, and the ventral beak becomes more atten- 

 uate while the greatest width of the shell, instead of being at or below the 

 middle, comes nearer the hinge-line. 



Beak. From being erect, straight, and relatively broad in the ventral 

 valve, at the outset, it becomes, at maturity, narrow, attenuate, and slightly 

 incurved toward the apex. 



Foramen. In the earliest observed stage, the foramen is a broad, 



triangular opening, covering nearly the entire cardinal area, reaching, but 

 not encroaching upon the apex of the valve. In subsequent stages of 

 development, this opening narrows with the narrowing of the beak, but, as 

 at no stage deltidial plates are developed, the contraction is due to the en- 

 croachment of the cardinal portions of the valve along the foraminal mar- 

 gins. The interesting fact of the persistent absence of deltidial plates 

 throughout the entire existence of the individual, may be interpreted as a 

 retention to maturity, of a character embryonic in allied species ; the small 

 size of the mature shell and the very slight incurvature of the ventral 

 beak also contribute to the embryonic expression of the species. 



