SILURIAN BRACHIOPODA 39 



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 elementary 

 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 margins thickened, and develops small and obscure 

 deltidial plates at its base, which at maturity leave the foramen elon- 

 gate and not circular. In respect to these features, the development of 

 the species is identical with that of R. indianensis. 



Plications. In the first observed stadium, only the umbonal area is 

 smooth, and from the analogy of R. indianensis, it would appear that the 

 initial growth-stage yet fails us. At a size of 2.5x2 mm., the number of 

 plications has increased from four to ten on the dorsal, and from three 

 (five ?) to eleven on the ventral valve, and this, added to a pair of ex- 

 tremely obscure plications near the cardinal margins, is the normal number 

 for maturity. 



RHYNCHONELLA WHITII, Hall, 1863 



PLATE IV, FIGS, i, 2, 4, 5 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 164, pi. 26, figs. 23-33. 



1879. 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 307, pi. 26, figs. 23-33. '882. 



Like Rhynchonclla ncglccta, this species is subject to very slight vari- 

 ations at maturity, and its specific expression is well marked, but a certain 

 embarrassment attends the first endeavor to separate the immature individu- 

 als from those of allied species. This, however, disappears with a careful eye 

 properly estimating the essential characters of the species. The earliest 

 stage of growth we have found measures 2.75 mm. in length by 2 mm. in 

 breadth, and from this size upward to that of 13x13 mm. all variations are 

 present. 



