DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 343 



This is a marked instance of reversion after the assumption 

 of certain adult features. 



incipient Form (Plate XVIII, figures 6, 6 a). The ex- 

 ample with which the present series opens measures .75 x 

 .5 mm. It is elongate sub- triangular, with the ventral beak 

 elevated and erect, the cardinal margins sloping for two-thirds 

 the length of the shell ; foramen triangular, slightly, if at all, 

 encroaching upon the apex, without deltidial plates, margins 

 thin ; dorsal beak rounded, inconspicuous. At one-third the 

 distance from the apex to the anterior margin, fine thread- 

 like plications appear, four upon the dorsal, and three (five ?) 

 upon the ventral valve. The median sulcus on the dorsal 

 valve is broader and deeper than any other, forming the em- 

 bryonal sinus, and is accompanied by a correlatively strong 

 plication on the opposite shell. 



Developmental Variations. 



General Form and Outline. -?- The form of the shell varies 

 from dimensions in which the length is one-quarter greater 

 than the width, to those of maturity when the width is 

 slightly greater than the length. The depressed, sub-spatu- 

 late embryo eventually becomes convex and deep. The em- 

 bryonal sinus and fold on the dorsal and ventral valves, 

 respectively, are never so prominent as in 0. indianensis, and 

 soon become lost, the former in a sulcus, and the latter in one 

 or more sulci upon the reversed fold and sinus of maturity. 



Beak and Foramen. The erect and acute beak of the ele- 

 mentary stages of growth becomes, at maturity, but slightly 

 incurved, and never procumbent on the dorsal umbo. The 

 foramen, at the outset triangular, subsequently has its mar- 

 gins thickened, and develops small and obscure deltidial plates 

 at its base, which at maturity leave the foramen elongate and 

 not circular. In respect to these features, the development 

 of the species is identical with that of 0. indianensis. 



Plications. In the first observed stadium only the um- 

 bonal area is smooth, and from the analogy of C. indianensis 



