189 



while there may be an outer calcareous layer to S. priscus, it has not been 

 detected. From the former species, which is Ordovician, it differs- not . 

 only in its small size, but its orbicular form ; it differs also in having, 

 radiating as well as concentric strise on the outer surface. It is much 

 smaller than Dr. Ami's S. Canadensis of the Utica shale. 



In one example of the ventral in this species the foramen is in the 

 umbo, but in the others it is in front of it. The ring around the inside 

 of the foraminal opening is never prolonged into a tube as in Siphonotreta. 



In re-examining the material from this horizon at Navy island, St. 

 John, I find that this species is present there also, but the surface mark- 

 ing are not well preserved ; however, the form and size of the shell, and 

 the foraminal opening, show it to be the same species. 



LEPTOBOLUS FLUMENIS, 11. sp. PI. XL figs, a f 



A narrowly ovate species, somewhat straight on the sides, and broadly Leptobolus 

 rounded in front. 



The ventral valve has an obtusely pointed beak, bent down at the apex. 

 Sides of the valve somewhat sharply sloped in the posterior half and 

 gently sloped in front. Interior. This shows an area nearly half a milli- 

 metre long, and a visceral callus extending about half the length of the 

 valve. The lateral ridges within the valve extend as far ; and in front of 

 them, reaching to within a sixth of the length of the valve from the front, 

 are a pair of arched vascular groves, not far from the margin of the valve ; 

 numerous external branches from these trunk grooves extend to the flat- 

 tened margin of the valve ; the individual length of these branches is 

 about equal to the width of the main groove. 



The dorsal valve is obtusely rounded at the hinge area, which is very 

 short, and here and at the sides the valve is strongly arched downward, 

 but is gently sloped down in front. Interior. In front of the linear 

 cardinal area is a pair of pits marking the insertion of the cardinal muscles. 

 The middle of the valve for more than half the valve's length has a flat- 

 tened band marking the progress of the central muscles during the 

 growth of the shell. At the front this band is about one sixth or one 

 eight of the width of the valve, and at its sides are faint prints of the 

 central muscles ; and at the front are two pairs of minute pits, one or both 

 of which marks the position of the anterior laterals. The place of the pos- 

 terior laterals is marked by a series of small pits near the margins in the 

 posterior third of the valve. In front of these pits are the strong arched 

 grooves left by the vascular trunks, which extend across the middle half of 

 the valve near its margins. 



