NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA GRANT 51 



Table 3. — Measurements of Atribonium gregeri 



Measurements, in millimeters 



Comparisons. — Atribonium gregeri is characterized by its rather 

 large size for the genus, and its globose shape. Adults of this species 

 are easily distinguished from all other known species of the genus by 

 their globose shape and lack of the flattened anterior that characterizes 

 all other species. Juveniles retain the flattened anterior, although not 

 as flat as in other species ; they can be distinguished by their more 

 convex valves and rather gentle curvature toward the commissure at 

 the anterior. 



Discussion. — This globose species retains the flattened anterior sur- 

 face that characterizes the genus Atribonium only in its juvenile stages. 

 As the shell grew, the increase in thickness was accomplished by in- 

 creasing the convexity of the valves, rather than by adding shell ma- 

 terial nearly perpendicular to the plane of commissure as in most 

 species of the genus. However, the normal atriboniid juveniles, and 

 the high flat uniplication of the anterior commissure and the numerous 

 weak costae identify it as a species of Atribonium, and distinguish it 

 from similarly globose but much more strongly costate species of 

 Coledium. Furthermore, despite the convexity of the anterior of A. 

 gregeri, the valves meet nearly in a plane, rather than at a relatively 

 sharp angle as in Coledium. 



Stainbrook (1942, text fig. 6) presented drawings of cross-sections 

 of A. gregeri showing the spondylium braced against floor of the valve 

 by two short lateral plates in addition to the median septum. My serial 



Fig. 9. — Scatter diagrams of the dimensions of Atribonium gregeri from all 

 localities listed for the species. 



