392 Capt. T. Brown on some new species of the genus Pachyodon. 



nearly parallel ; anterior side short and rounded ; posterior side 

 elongate, and obliquely subtruncate from tbe hinge line, termi- 

 nating below in a short, slightly acuminated curve ; surface 

 rather smooth, with a few, distant, transverse, shallow grooves. 

 Ironstone shale at Low Moore, Yorkshire. 



7. Pachyodon Dawsoni. PI. XV. fig. 10. 



Orbicular ; umbones central, large, produced and remote ; sur- 

 face nearly smooth, with only a few nearly obsolete concentric 

 wrinkles ; thickness equal to more than half its diameter. 



Found in the ironstone shale at Low Moore near Bradford, 

 and is in the cabinet of Mr. S. Gibson. 



8. Pachyodon nanus. PI. XVI. fig. 1. 



Smooth, posterior side elongated and obliquely subtruncate 

 above, subacute below, anterior side rounded; umbones pro- 

 duced and rounded ; hinge line arcuated. 



Coal shale at Middleton, near Leeds. 



9. Pachyodon Rhindii. PI. XVI. fig. 2. 



Subacute at both extremities ; basal line considerably arcuated, 

 rather produced opposite the umbones; posterior side turned 

 slightly upwards; umbones subcentral, prominent, and very 

 close ; hinge line curved ; surface with transverse, shallow, irre- 

 gular wrinkles. Length equal to two-thirds of its breadth. 



Coal shale, Polmont, Stirlingshire. 



10. Pachyodon Amygdala. PI. XVI. fig. 3. 



Inflated, anterior side rounded; posterior side acuminated, 

 with an acute beak-like termination ; umbones rather obtuse and 

 remote ; basal line considerably arcuated ; surface with many 

 irregular acute wrinkles. 



Ironstone shale. Low Moore, Yorkshire. 



11. Pachyodon exoletus. PI. XVI. fig. 4. 



Shell transversely elongate, its breadth about double its length ; 

 surface quite smooth ; umbones blunt, placed near to the anterior 

 side, which is round ; posterior side acuminated and subacute ; 

 Jiinge line slightly arcuated, basal line nearly parallel ; thickness 

 somewhat more than half its length. 



Ironstone shale. Low Moore, near Bradford. 



12. Pachyodon dubius. PI. XVI. fig. 5. 



Subovate, both sides rounded; umbones slightly produced 

 and rounded ; hinge and basal lines arcuated ; surface with nearly 

 obsolete, irregular, concentric wrinkles. 



Coal shale near Newcastle-on-Tyne. 



