150 



figured for that species, while found on the dorsal valve of ours, do not 

 occur in the ventrals, and it is the ventral of his species that Mr. Walcott 

 figures ; however, he describes the species as having very fine costae, 8 in 

 the apace of 3 mm. ; our species has 4 or 5 in the same space, but young 

 shells may have 6. 



Form. The valves differ considerably in their proportions, but this 

 appears to be due to distortion after they were buried ; as now found, 

 the valves range from those in which the length and width are equal, to 

 others in which the width is a half more than the length. Taking an 

 average of a number of valves as shown in the accompanying table, the 

 width appears to be one-quarter greater than the length, but less than 

 this in the ventral valves. 



VALVES of Billingsella retroflexa from sandstone of E. 2 (a 1) at 



at McFee's (Young point.) 

 (The dimensions in this table are in millimetres.) 



The following are measurements of valves from the sandy shale below 

 the sandstone bed at Young point. 



