DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 349 



large and two quite small, making one broad and one 

 narrow sulcus upon the fold. The phenomenon may be due 

 to the strongly developed tendency of the embryonal sinus to 

 maintain its continuity with a median sulcus even at the ex- 

 pense of the symmetry of the shell. 



Initial SheU (Plate XVII, figures 17, 17 a). The initial 

 shell in this series of Camarotoechia indianensis measures .65 

 mm. in length by .54 mm. in width. It is broadly ovate or 

 sub-pyriform in aspect, convex posteriorly, and depressed 

 toward the anterior margin. Ventral valve with the umbo 

 prominent, the beak elevated and erect, with the apex rounded ; 

 cardinal margins rapidly sloping. Foramen sub-triangular, 

 apical portion broader than usual in the incipient stages of 

 plicate shells ; margins not thickened ; deltidial plates absent. 

 Dorsal valve with a rounded, inconspicuous beak. Surface of 

 both valves quite smooth. A median depression is noticeable 

 on the dorsal valve near the anterior margin, making this 

 margin sinuate. This embryo is the smallest that has been 

 found for any of the series of Rhynchonellidse ; and not only 

 on account of its minuteness, but also from the entire absence 

 of plications on its surface and from the elementary character 

 of the cardinal area, it may be regarded as the actual elemental 

 or initial shell. 



Developmental Variations. 



General Form and Outline. The adult variations from the 

 normal noticed above seem to be in most instances, and prob- 

 ably would prove to be in all, preceded by well-defined 

 embryonic series leading up to them. This must be the 

 case, as the character of these variations, i. e. variation in 

 the number of plications on the median portions of the shell, 

 is such that they cannot be assumed after the attainment of 

 the adult condition, as is possible in certain other forms of 

 variation. But it is not to be assumed that the conformation 

 of the embryo which eventually produces any of these results 

 manifests them in the earliest stages of the growth of the 

 shell ; rather, that the shells, under whatsoever variations at 



