CONTRIBUTIONS TO PALEONTOLOGY. 133 



GENUS SPONDYLOBOLUS ( M'Coy). 



** Generic character. Suborbicular, slightly narrowed towards 

 '' the indistinct short hinge-line, nearly equivalve, flattened. 



" Small valve with a slightly excentric apex ; beneath which, 

 " on the interior, the substance of the valve is thickened into 

 " a wide undefined boss. Opposite valve slightly longer, from 

 " the apex being perfectly marginal and slightly produced, 

 " channelled by a narrow triangular groove below, the anterior 

 " end of which is flanked by two very prominent, thick, conical, 

 " shelly bosses representing hinge-teeth : substance of the valve 

 *' thick, testaceous, not glossy, minutely fibrous, but not dis- 

 " tinctly punctated under a lens of moderate power, except by 

 " the ends of these fibres." 



AVhether the characters here given are such as will bear some extension 

 by the study of a larger number of species ; and whether, if somewhat 

 extended, they may not include some of those forms like Obolella, I 

 shall not discuss at the present time. With the exception that the apex of 

 the small valve is described as slightly excentric, the description corresponds 

 more nearly than that of Obolella with the shell before me. 



OBOLELLA? POLITA. 



plate VI. FIGS, ir - 21. 



Obolus apollinus : Owen (not Eichwald), Loc. cit. 



Geological Report of Wisconsin, Vol. i, p. 21 and p. 435. 

 Lingula? polita ; Annual Geological Report, Wisconsin, 1860, p. 24. 



Shell small, short ovate, length and breadth nearly or quite 

 equal ; the greatest width near the front, which is broadly 

 rounded. Yalves moderately convex, with prominent umbones, 

 somewhat inequivalve : beaks obtuse, one usually a little trun- 

 cate or emarginate. Surface smooth, sometimes glabrous, with 

 concentric striae : shell calcareous?, thick, fibrous or lamellose. 

 The interior of the ventral(?) valve has a broad somewhat 

 cordiform subcentral duplicate muscular impression, with a raised 

 margin. From the centre beneath the beak extends a low flatly 

 rounded ridge, which reaches into and partially divides the mu- 

 scular impression. On each side of this low ridge is a nearly flat 

 slightly elevated plate or thickening of the shell, which, extending 

 forward, is continued in its outer limbs in a raised border nearl}^ 

 around the muscular impression ; giving to the whole a broad 

 spatulate form, leaving a space in front where the shell is marked 



