137 



Sculpture. This consists of laruellose concentric ridges ; toward the 

 front margin of the valve there are about 8 in the space of a millimetre ; 

 the ridges are more closely set toward the sides and at the mid length of 

 the valve ; they anastomose mo: e or less, and sometimes two or three run 

 into another ridge, and are abruptly cut off. 



Size. Length of ventral 9 mm. ; width 8 mm. ; depth about 1 J mm. 

 The dorsal is of the same depth, and is 1 mm. shorter. 



Horizon and locality. Sandstone and grit layers of Assise E 1 6, and 

 in the shales of E 1 c at Dugald brook, Escasonie, N.S. 



This species differs from 0. cequiputeis in the weakness of ' the anterior Contrasted 

 scars of the central group, in the sharper sculpture and the strong pro- 

 longed lateral septa somewhat curved at the anterior end. From 0. 

 discus in the advanced position of the middle one of the three minor scars 

 of central group and in the marked lateral septa, as well as the project- 

 ing umbo of the ventral valve. 



This species departs from typical Obolus in the projecting umbo, strong 

 lateral septa and advanced position of the whole central group of muscle 

 scars in the dorsal valves. 



The following are dimensions of the valves in this species : 

 VALVES of Obolus triparilis from Assise E. 1 6, 



