44 G. P. MATTHEW ON THE 



O. Ingricus, O. Siluriciis and O. antiquissimus, which we have not yet seen, but which, 

 according to the descriptions, differ little from the species described below." 



" Horizon and Locality. — Shells of Obolus are found by millions in the lower sand- 

 stones [Cambrian] of the Silurian system of the north of Russia, and there constitute entire 

 beds. Their valves not being articulated are easily disunited, and their fragments lying 

 in the plane of the beds give the rock the appearance of a micaceous sandstone. These 

 shells mark a fixed horizon which serves as an excellent guide to the geology of the 

 entire Baltic Coast * ^ * but it is remarkable that notwithstanding their extreme 

 abundance they have never been found on the opposite coast of the Baltic, neither in 

 Sweden nor Norway, notwithstanding that sandstones exist there parallel to those of 

 Russia, and inferior in position to the limestone with Amplms expansits, etc. According 

 io M. Eichwald, who has particularly studied the distribution [of Obolus], two species, 

 O. anliqui ssumus and O. Siluriens, occur in chloritic limestones, a little higher up than the 

 sandstones." 



E. de Verneuil describes Obolus ApoUinis as follows : — ' 



" Subequivalve equilateral, very flat, orbicular or slightly transverse, having a corneous 

 test, varnished, brilliant, of brown or blackish color. Edges thin, sharp, fragile, and pre- 

 senting neither inflections nor sinuosities, apex but slightly marked and quite terminal ; 

 surface covered with irregular longitudinal strise, scarcely visible, similar to those of cer- 

 tain Lingulse, and in part concealed by transverse striœ crossing them. 



" Dorsal [ventral] valve at the summit very obtuse, having a flat cardinal beak, trans- 

 versely striated, at the middle of which one finds an elongated canal, destined without 

 doubt to receive the muscle of attachment, which, as in the Lingulas, passed between the 

 summits of the two valves. This same valve is besides furnished on the inside with a 

 median ridge, terminating in a point as in the Lingulas, and two lateral ridges much 

 more prolonged. 



" The ventral [dorsal I valve is a little shorter than the other, more rounded at the 

 hinge, with a cardinal area very large, having a flat surface and not channelled. In the 

 interior several impressions of muscles have been observed. 



" Dimensions. — Length, 10-12 mm. ; width, 12-14 mm." 



A species similar to this, but not identical, has been found in the shales of Division 

 3 of the St. John Group, viz. : — 



Obolus refulgens, n. sp. (PI. XII, figs. 6 a-d.) 



Entangled with the hydrosomes of Dictyonema and scattered through the shales in 

 which that graptolite occurs there are numerous examples of an orbicular brachiopod, 

 in many respects resembling the Russian species of Eichwald described above, which 

 gave its name to the " Ungulite grit " of Russia. 



Shell oblately orbiciilar, lenticular, edges thin, sharp, flat. Test corneous ; surface 

 brilliant. 



' Op. cit., p. 291. 



