162 



AMERICAN JOURNAL 



A, 



Fig. 38. 

 Fig 38. Obolus JJavidsoni, Salter; after Davidson. 4, neural valve; the 

 central scar is that of the pedicel muscles. 5, htemal valve. A, post-adduc- 

 tors. B, sliding muscles. C, anterior adductors. 



Genus OBOLUS, Eichw. 



Obolus, EichAvald, Zool. Spic. vol. i, p. 274, 1829 ? 



Ungula, Pander, Beitriige zur Geog. Russ. Reich. 1830. 



UnguUtes, Pander, Herman, ii, p. 132. 



Aulo)iotreta, Kutorga, Ueb. die Siph. Verb. K. rain. Ges. fur 

 1847. 



Shell subequivalve, orbicular, non-;irticulated valves united 

 to each other by the muscles. 



Neural valve most convex, with a short beak, flat hinge mar- 

 gin and false area, over which the lines of growth pass without 

 interruption. The cardinal border is longitudinally grooved for 

 the passage of a peduncle. H^mal valve shorter than the other, 

 slightly convex or quite flat, without a prominent apex. The 

 hinge line passes, in a rounded arch, insensibly into the lateral 

 margin ; the cardinal border is flat, thickened and transversely 

 striated, but not sulcated. Structure calcareo-corneous. Exter- 

 nal surface smooth or concentrically wrinkled. The neural valve 

 is marked by a mesial ridge extending half its length ; four 

 small oval muscular scars are situated, one on each side, near the 

 cardinal angles {retractores posteriores), the other two near the 

 apex, on each side of the mesial ridge {adductores posteriores), 

 and the scars of the anterior adductors are somewhat elongate, 

 with their pointed terminations converging anteriorly in the mid- 

 dle of the valve. 



Ti/pe. Obolus apollinls, Sch., Dav, Int. Br. Brach. p. 136, 



pi. ix, fig. 280—84. 

 Fossil in the Silurian beds of Russia. 



Genus OBOLELLA, Billiugs. PI. 6, fig. 7. 

 Obolella, Bill., Pal. Fos. Can. Geol. Surv. vol. i, 18(31, p. 7. 

 Shell ovate or subquadrate, convex or compressed. Neural 



