OF CONCHOLOGY. 163 



valve with a more or less minute false area, usually grooved for 

 the passage of the peduncle. Htemal valve with or without an 

 area. Lateral scars near the margin of the cardinal border ; 

 scars of anterior adductors differing from those of Obolus in 

 diverging instead of converging anteriorly. 



Ti/pe. OhoJella chromatica, Bill., Pal. Fos. Can. Geol. Sur- 

 vey, vol. i, 1861, p. 7, fig. 7 a, b, c, d. 



Subgenus Kutorgina, Bill. 

 Kuforgina, Bill., Pal. Foss. Can. Geol. Survey, 1861, p. 69. 



Hinge line straight, nearly as wide as the shell, sides nearly 

 straight, meeting the cardinal border at an obtuse angle. 



Neural valve with an area and foramen. A pair of subcentral, 

 oval muscular impressions, but no vestiges of lateral scars. 

 Externally radiately striate. 



Ti/pe. Oholella [Kutorgina) cingulata, Bill., Pal. Foss. Can. 

 Geol. Sur. p. 9, fig. 8, 9, 10. 



This subgenus differs from the typical form by its straight 

 hinge line and striations, as well as the absence of the lateral 

 scars ; it resembles an Ortliisina externally. 



Subgenus Monobolina, Salt. 



Monobolina, Salter, Mem. Geol. Surv. Great Britain, vol. iii, 

 p. 334, 1865. 



Shell resembling Obolus; broad; external surface radiately 

 striated ; muscular scars united closely along the central line. 



Ti/pe. 3Ionobolina plumbea. Salt., Siluria 2nd Ed. p. 50, 

 Fos. 8, fig. 1, 1859. Dav., Sil. Brach. p. 61, pi. iv, fig. 

 20, 27. 



Fossil in the Silurian formation of Gt. Brit. 



The Obolus f Salteri, Dav., Sil. Brach. pi. iv, fig. 28, 29, ap- 

 pears to be more nearly related to the Lingulas with an internal 

 mesial ridge than to any of the OboUnce. 



Genus SPONDYLOBUS, McCoy. 



Spondylobus, McCoy, An. Nat. Hist, viii, 2, p. 407. 



Suborbicular, slightly narrowed toward the short indistinct 

 hinge-line ; nearly equivalve, flattened. Haemal valve with a 

 slightly excentric apex, beneath which, on the interior, the sub- 

 stance of the valve is thickened into a wide undefined boss. Op- 

 posite valve slightly longer, from the apex being perfectly mar- 



