NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA GRANT 49 



B 



F G H 



Fig. 8. — Atribonium gregeri (Branson), Cedar Valley Limestone, Iowa, trans- 

 verse sections, X6, USNM 141857 cut perpendicular to surface of brachial valve 

 to show true height of camarophorium (hence long crura in fig. G, H) original 

 length : 9.9 mm., all measurements from pedicle valve beak. 



A. 0.05 mm.: camarophorium thick. B. 0.1 mm.: low cardinal process. C. 0.2 

 mm. D. 0.6 mm. : dental plates detached from walls. E. 0.8 mm. : hinge plate 

 detached from walls. F. 1.1 mm. : intercamarophorial plate gone, crural bases 

 apparent. G. 1.3 mm. : cut at angle to camarophorium, apparently thickening 

 spoon and showing crura. H. 1.5 mm. : spondylium gone. Crura disappear at 2.0 

 mm., camarophorium at 3.1 mm. 



Growth. — This globose species exhibits a pattern of growth some- 

 what different from that of other species of Atribonium. Points rep- 

 resenting the length- width ratio (fig. 9) cluster narrowly along a 

 straight line, which is normal for the genus. However, points repre- 

 senting the length-thickness ratio cluster along a curve of gradual 

 deceleration, where other species normally cluster near curves of ac- 

 celeration (figs. 6, 10). A. gregeri is globose at an early stage, when 

 the shell is only 6 or 7 mm. long. Subsequently the rate of increase in 

 thickness declines slightly, compared with the rate of increase in length. 

 Projection of this curve toward low values on the length-axis neces- 

 sitates a reverse flexure, indicating that short shells probably were very 

 flat as in other species of the genus. Projection of the length-width line 

 also necessitates a slight change in declivity in the low values, but this 

 is normal. 



