70 MEMOIRS OF THE STATE MUSEUM 



MERISTINA NITIDA, Hall, 1843 



PLATE VII, FIGS. 6-10 



, Hall. Twenty-eighth Ann. Kept. State Mus. Nat. Hist., p. 160, pi. 25, figs. 1-7. 1879. 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 300, pi. 25, figs. 1-7. 1882. 



Meristina nitida is a very abundant and characteristic fossil in the 

 Niagara fauna of central Indiana, reaching a much greater development 



both in size and numbers than in the New York outcrops of the formation. 

 The individuals vary in size from a length 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 represent- 

 ing 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 life 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 ap- 

 proaching maturity. 



SPECIFIC CHARACTERS 



Mature Form (plate 7, figs. 9, ga, 9^). Shell broadly subpentagonal 

 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 truncated, the circular foramen lying in a vertical plane. 

 Cardinal 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 valve 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 striae and a few conspicuous lines 

 of growth toward the anterior margin. Average individuals measure 2OX 

 16 mm., large examples not infrequently 25x21 mm. 



