149 



surface about as numerous as those described by Billings for 0. Jesinata, 

 the other with much closer and finer ribs like 0. orientalis Whitfield. 

 But on sorting the valves into ventrals and dorsals it was found that the 

 valves with narrow ribs were ventrals, while those with wide ribs were 

 dorsals. It might be supposed that this peculiarity would prevent the 

 valves from fitting close together at the margins, but such a difficulty 

 could have been obviated by a special smoothing of the edges of the valves. 



The diversity in the width of the ribbing is most marked in the middle 

 stages of growth ; in both the larval and senile stages the valves were 

 smoother. The spaces between the ribs in the dorsal valve are wider 

 than the ribs ; they are flattened and traversed by two, sometimes three, 

 low fine, slender, thread-like ridges, parallel to the main ridges ; on the 

 lateral slopes the spaces between the ribs have only one of these low 

 threadlike ridges. 



This species has in the ventral valve 5 ribs in the space of 3 mm. and 

 the dorsal has four in the same space, counting the thread-like ridges as 

 well as the ribs. 



Size Length of the ventral, 18 mm. ; width, 20 mm. ; depth at the 

 umbo, about 4 mm. The dorsal is about 2 mm. shorter and has a depth 

 of 4 mm. about the middle of the valve. 



Horizon and locality. The original examples were from a bed of gray Geological ag 

 sandstone at McFee's (Young point), near George river station, Cape and localit y- 

 Breton, where it occurs with Lingulella Selwyni and was collected by 

 Messrs. Weston and Robert, of the Canadian Geological Survey. The 

 species also occurs, but of smaller size, in the sandy shales that underlie 

 this sandstone. The sandstone in places is quite calcareous, and becomes 

 as coarse as a grit. The fossils are in the condition of casts that have 

 weathered out in this, now porous rock. These beds are tentatively 

 assigned to Assise E 2. a. of the succession in the gorge at Dugald brook. 



Billings described the dorsal valve of his species (B. festinata] as being 

 nearly flat. Such a description does not apply to B. retroflexa, in which 

 this valve has the usual convexity of an Orthid. But the ventral valve 

 is nearly flat, except near the umbo, though it slopes down to the hinge 

 line on each side, owing to the elevation of the umbo. 



B. orientalis of Whitfield has close, narrow ribs in the ventral valve 

 (the dorsal is not known) like B. retrqftexa, and might be compared with 

 laterally compressed examples of this species, but the print of the ad- 

 ductor muscles in that species is much larger than in this. 



B. transversa Walcott is not unlike longitudinally compressed examples Compared 

 of this species (B. retrqflexa), but the fine ribs between the coarse ones, species. 



