129 



5. On some of the Fossils of the Arisaig series of rods. Upper 

 Silurian, Nova Scotia. 



Lamellibranchiata. 



The genera Qrthonota (Conrad) 1841, Sanguinolites (McCoy) 1844, 

 and Goiiiophora (Phillips) 1848, seem to be all closely related to each 

 other. The following references to these genera are, therefore, to be 

 regarded as merely provisional. 



Ortiioxota venusta. (N. sp.) 



Plates, fig. 1. 



Description. — Shell transversely elongate ; length about four times the 

 lieight ; dorsal and ventral margins neai'ly parallel, slightly diverging 

 posteriorly ; beaks situated at about the anterior fourth of the length. 

 Behind the umbones the hinge-line is straight, and extends nearly the 

 whole length of the shell. In front of the umbones it is also straight, but 

 at a somewhat lower level than behind. The anterior extremity is nearly 

 vertically truncated for a short distance below the hinge-line, then rounded 

 into the ventral margin ; the latter appears to be slightly convex. The 

 posterior extremity is not well preserved in any of the specimens examined, 

 but appears to be nearly vertically truncated in the upper two-thirds, and 

 rounded below. The valves are moderately convex ; an obscure rounded 

 angulation extends from the umbones backwards and dowmwards to the 

 lower posterior angle. Above this the shell is compressed, or even gently 

 concave. A similar rounded angulation runs from the umbones to the 

 lower anterior angle. 



The surface, at the anterior extremity, exhibits usually eight or nine 

 narrow but strongly elevated ridges. Before the beaks these descend, 

 nearly vertically from the hinge-line, to the anterior umbonial slope, over 

 which they bend with a narrowly rounded curve, and then slope back- 

 wards and downwards in straight parallel lines towards the ventral mar- 

 gin. Three or four of the most anterior ridges seem to die out on reach- 

 ing the margin. The others become parallel to the margin for a short dis- 

 tance, and then turn upwards to the dorsal margin. Above the posterior 

 umbonial ridge, there are three or four shallow grooves, which originate 

 in a point near the umbones, and gradually radiate backwards to the 

 upper half of the posterior extremity. The concentric ridges are more 

 acute and strongly elevated on the anterior than they are on the posterior 

 part of the shell, 



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