138 



OBOLUS DISCUS, n.sp. PI. VIII, figs 3 a-d. 



Obolus(E.) Evenly lenticular with the beak of the ventral slightly projecting, 

 described. Length and breadth about equal. 



Ventral valve rather flat, with an appressed umbo, scarcely elevated 

 above the cardinal line, and but slightly projecting beyond the general 

 circular contour of the valves. Interior not known. 



The dorsal valve is evenly sloped toward the margins, which are some- 

 what flattened. A median sulcus appears in the back of the valve, near 

 the hinge, and becomes wide and shallow towards the front ; it extends 

 two-thirds of the length of the valve. Interior. The cardinal area is 

 about one-seventh of the length of the valve, and has a wide pseudo-del- 

 tidium, transversely striated ; this area is twice as wide at the margin of 

 the visceral cavity as it is at the apex of the shell ; the cardinal borders 

 also are wide and are traversed by strife that turn outward toward the 

 lateral margin of the valve. Lengthwise along the centre of the valve is 

 a raised callus that carries the central muscles ; it is marked by a median 

 and two lateral furrows, that extend about half of the length of the valve ; 

 at the front of the callus are the central group of scars (arranged in quin- 

 cunx?^ of which the two posterior are much larger than the others. The 

 two anterior scars are connected with the two posterior by a faint thread- 

 like ridge. The small central scar is about midway between the four. 



Sculpture. This consists of irregular, anastomosing, beaded ridges. 

 Over some parts of the valves these markings are hardly distinguishable, 

 and the surface appears granulated. 



Size. The largest ventral observed was 10 mm. long, and the largest 

 dorsal 9 mm.; the width is about 9 mm., and the depth 1^ mm. 



Horizon and locality. Sandy layers in the shale of Assise E. le at 

 Dugald brook, Escasonie, N.S. Not rare. 



This differs from the next in its somewhat smaller size and more exactly 

 lenticular form of the dorsal valve ; in the less prominent umbo of the 

 ventral, as well as the more irregular sculpture and the flattening of the 

 dorsal. The umbo of the dorsal seems to have been shortened. 



The material for this species is scanty, and the following are dimen- 

 sions &c. of the valves examined : 



