162 



AMERICAN JOURNAL 



Fig. 38. 

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

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

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



Genus OBOLUS, Kichw. 



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



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



Ungidites, Pander, Herman, ii, p. 132. 



Aulonotreta, Kutorga, Ueb. die Siph. Verb. K. min. Ges. fiir 

 1847. 



Shell subequivalve, orbicular, non-articulated 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. Haemal valve shorter than the other, 

 sliglitly 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 posterm^es), 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 apollinis, Sch., Dav. Int. Br. Brach. p. 136^ 



pi. ix, fig. 280—84. 

 Fossil in the Silurian beds of Russia. 



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

 Obolella, Bill, Pal. Fos. Can. Geol. Surv. vol. i, 18G1, p. 7. 

 Shell ovate or subquadrate, convex or compressed. Neural 



