341 



manner somewhat similar to that of casts of the interior of the semis 

 Cleidophorus (Hall), but sloping backwards instead of forwards. There 

 is a line also running round parallel with the ventral margin and near it, 

 'resemaling the paUial line of the lamellibranchiate shells. The form is 

 someyhat variable, the ventral margin not having always the same amount 

 of convexity. Both sides of the shell are gently convex. 



Length from 8 to 16 lines ; width of the shell from the beaks to the 

 ventral margin, about half the length. 



Locality and Formation. — Counties of Leeds and Grenville ; Calcifer- 

 ous formation. 



Collector. — E. Billings. 



RiEBIRIA ? LONGIUSCUIiA. (N. Sp.) 

 Pig, 321 



Description. — The specimen on which this species is founded is oblong- 

 ovate, the dorsal and ventral margin in the posterior two-thirds nearly 

 parallel, the anterior extremity uniformly rounded, the posterior third 

 apparently narrowed, as the ventral margin slopes upwards, as if to form 

 a termination like that of H. ealcifera. The dorsum is straight from the 

 beaks backwards. The portion in front of the beaks nearly straight, 

 lower than the portion behind. The orifice beneath the beaks is obscurely 

 seen in the specimen. Both sides of the shell are gently convex, and 

 from the beak there is a wide shallow concavity extending backwards and 

 downwards to the ventral margin. The slope of the shell extends all 

 round. The dorsum is entire and narrowly rounded, sub-carinated. 

 Surface unknown. 



The specimen when perfect was apparently about 16 lines in length 

 and 7 in width. 



I have some doubts whether this species is distinct from B. ealcifera, 

 as the two were found together in the same beds, but this one is extremely 



rare. 



Locality and Formation. — Counties of Leeds and Grenville ; Calcif- 

 erous formation. 



Collector. — B. Billings. 



