BRACHIOPODA. 337 



SUPPLEMENTARY NOTE ON THE GENUS OBOLUS, VON EICHWALD. 



The preceding discussions of the inarticulate genera (ending on page 184) 

 were received from the printer in March, 1890. Since that date an impor- 

 tant contribution to our knowledge of the genus Obolus has been made by 

 A. MiCKWiTz,* whose studies are based upon finely preserved material from 

 Joafall, near Jegelecht, Esthonia. 



Students of the brachiopoda will appreciate the author's statement that "this 

 genus, notwithstanding the exceeding abundance of its shells in the upper layers 

 of the Ungulitensandstein of our Cambrian formation, from Balfischport to the 

 banks of the Sjas, and in spite of its early discovery, has been hitherto as good 

 as unknown." The details of the internal structure of both valves are worked 

 out with such a degree of elaboration, that we can do no less than give in 

 this place the author's diagnosis of the genus and copies of his accompanying 

 illustrations, observing that the genera Aulonotreta, Kutorga, and Schmidtia, 

 Volborth, are here regarded as synonymous with Obolus : 



" Shell nearly equivalve, equilateral, depressed or slightly convex ; outline 

 circular or somewhat elongate longitudinally or transversely, in some species 

 subtriangular or elongate-quadrate. 



" Shell-substance calcareo-corneous, structure laminated ; surface lustrous, 

 with concentric and radial striae varying to deeply incised transverse folds and 

 radial ribs. Color of the shell dark greyish-blue to black ; when in process of 

 decomposition, whitish to dark brown-red. Anterior and lateral margins thin, 

 sharply angled and fragile, lying in the plane of the greatly thickened cardinal 

 margin. The cardinal area lies in the plane of the margins, in the larger valve 

 being somewhat triangular on account of the elevation of the beak, and divided 

 equally by the pedicle-groove ; in the smaller valve rounded at the apex, and 

 in both striated parallel to the base, and grooved from apex to base by the 



*Voilautiye Mitlheilung iiber das u^eiius Oeolcs, Eiihwald : Melanges geologiques et piilecinlolcgiqnes 

 tires du Bulletin de rAcademie Impeiiale des Sciences de St. Petersbourg, Tome I. (Read October 9th, 1890.) 



