344 STUDIES IN EVOLUTION 



it would appear that the initial growth-stage is still wanting. 

 At a size of 2.5 X 2 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 extremely obscure plications near the cardinal mar- 

 gins, is the normal number for maturity. 



Camarotoechia Whitii Hall, 1863. 



(Plate XVIII, figures 1, 2, 4, 5.) 



Rhynchonella Whitii Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. 



Nat. Hist., p. 184, pi. 26, figs. 23-33, 1879. 

 Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 307, pi. 26, 



figs. 23-33, 1882. 



Like Camarotoechia neglecta, this species is subject to very 

 slight variations at maturity, and its specific expression is 

 well marked, but a certain embarrassment attends the first 

 endeavor to separate the immature individuals 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 found measures 2.75 mm. in 

 length by 2 mm. in breadth, and from this size upward 

 to that of 13 X 13 mm. all variations are present. 



Specific Characters. 



Mature Form (Plate XVIII, figures 2, 2 a, 2 h). — Shell 

 transversely sub-elliptical; length and width about equal. 



Ventral valve shallow; beak high, acute, somewhat atten- 

 uate, with the apex slightly incurved, but not concealing the 

 triangular unclosed foramen which reaches entirely across the 

 cardinal area. At its apex the foramen encroaches slightly 

 upon the umbo, and is narrowed somewhat toward the base 

 by the imperfectly developed deltidial plates. A median 

 depression makes its appearance at about one-third the dis- 

 tance from the umbo to the anterior margin, and soon 

 develoj^s into a deep sinus with sharply sloping sides. 



Dorsal valve deeper and more gibbous ; beak inconspicuous, 



