DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 369 



normal seems to have retained through all stages of growth 

 the proportions of the normal embryo. 



Beak and Foramen. The erect, straight, sub-acute beak 

 of the incipient 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 mar- 

 gins meet in such a manner as to leave the foramen sharply 

 acute below, and sub-triangular. By their subsequent up- 

 ward growth arid more complete union the foramen becomes 

 circular, the lines of symphysis with the valves still remain- 

 ing 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. 



Atrypina disparilis Hall, 1852. 

 (PLATE XIX, figures 17-23.) 



Ccelospira disparilis Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. 



Nat. Hist,, p. 162, pi. 25, figs. 39-43, 1879. 

 Hall. Eleventh Ann. Kept. 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, several hundred immature individuals 

 have been found, the youngest of which has dimensions of 

 2.5 mm. in length by 2 mm. in width, the greatest size at ma- 

 turity being 6.5 mm. in length by 6 mm. in width. The species 

 being in its mature size quite small and in its surface feat- 

 ures quite simple, it does not afford such scope for variations 

 through the later embryonic 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. 



24 



