64 MEMOIRS OF THE STATE MUSEUM 



The elongate variation from the normal seems to have retained through all 

 stages of growth the proportions of the normal embryo. 



Beak and Foramen. The erect, straight, subacute beak of the incip- 

 ient shell, in later growth becomes rounded and slightly arched or incurved. 

 The cardinal area in all stages of development is, however, high, exceptions 

 being made for the more extreme cases of obesity, where the deltidial 

 plates may be concealed, but the foramen is always exposed, and the beak 

 is never procumbent on the opposite umbo. The plates arising from the 

 base of the thickened foraminal margins, meet in such a manner as to leave 

 the foramen sharply acute below, and subtriangular. By their subsequent 

 upward growth and more complete union, the foramen becomes circular, the 

 lines of symphysis with the valves still remaining thickened. 



In occasional instances, a probable slight displacement of the plates 

 outwardly along the median suture, gives them the appearance of sloping 

 away from the median to the lateral sutures. 



CcELOSPIRA DISPARILIS, Hall, 1852 

 PLATE V, FIGS. 17-23 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 162, pi. 25, figs. 39-43. 



1879. 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 303, pi. 25, figs. 39-43. 1882. 



Although this species is one of the less abundant members of the 

 Waldron fauna, we have found several hundred immature individuals, the 

 youngest of which has dimensions of 2.5 mm. length by 2 mm. width, the 

 greatest size at maturity being 6.5 mm. length by 6 mm. width. The 

 species being, in its mature size, quite small, and, in its surface features, quite 

 simple, it does not afford such scope for variations through the later embry- 

 onic stages as many of its associated species and hence, it will be noticed, 

 that in surface sculpture, a permanency of character is retained through all 

 stages of growth. 



