DEVELOPMEXT OF SOME SILURIAN BRACIIIOPODA 375 



the New York outcrops of the formation. The individuals 

 vary in size from a lengtli of 2.5 mm. and a width of 1.75 

 mm., to a length of 25 mm. and a width of 22 mm., and the 

 series representing these variations is, on account of the 

 abundance of specimens, very complete within these limits. 

 It is, however, a noticeable feature of the species that in 

 most respects, except size, the characters of maturity are 

 assumed early in the hfe of the individual, and as the form 

 is essentially devoid of surface sculpture, the interest in 

 its development rests to a larger degree than usual upon 

 abnormalities in individuals either mature or approaching 

 maturity. 



Specific Characters. 



Mature Form (Plate XXI, figures 9, 9 a, 9 b}. — Shell broadly 

 sub-pentagonal to ovoid; beaks extended and more or less 

 prominent. 



Ventral valve with the greatest convexity at about one- 

 third the length of the shell in front of the beak. Beak 

 arched, incurved over the dorsal valve; apex evenly trun- 

 cated, the circular foramen lying in a vertical plane. Cardi- 

 nal slopes extending for more than one-half the length of the 

 shell. A low median depression is noticeable on the younger 

 portions of the valve. 



Dorsal v<ilve with a similar convexity; beak incurved and 

 concealed. A very low and inconspicuous median elevation 

 corresponds with the depression on the opposite valve. 



Surface smooth or with fine concentric strife and a few 

 conspicuous lines of growth toward the anterior margin. 

 Average individuals measure 20 x 16 mm., large examples 

 not infrequently 25 X 21 mm. 



Variations in Outline. — Two very distinct groups of forms 

 are evident in this species, in one of which, («) normal, the 

 outline of the mature shell is obcordate or sub-pentagonal. 

 When immature, the anterior margin is evenly circular, but 

 in all cases the proportion of length to width is essentially 

 the same. Probably five-eighths of the specimens found 

 belong to this group. 



