Capt. T. Brown on some new species of the genus Pachyodon. 393 



13. Pachyodon subtriangularis. PL XVI. fig. 6. 



Subtriangular, rather inflated; umbones very prominent, re- 

 mote, being nearly a quarter of an inch apart ; hinge line almost 

 parallel ; basal line with an undulation ; both sides rather 

 abruptly sloping ; surface smooth, with a slight elevation towards 

 the umbones. 



Ironstone shale at Coalbrook Dale. 



14. Pachyodon Smithii. PI. XVI. figs. 7, 8. 

 Subtriangular, rather inflated ; umbones subcentral, prominent 



and rounded, inflected and quite close; anterior side rounded, 

 posterior side subacute ; sm'face with transverse, rather deep irre- 

 gular wrinkles ; breadth about a third more than its length. 

 Ironstone shale at Sheden. 



15. Pachyodon Embletoni. PI. XVI. fig. 9. 

 Subtriangular, anterior side short and rounded; umbones 



placed much to one side, obtuse above, beaks inflected and sharp- 

 pointed; hinge line considerably arcuated; posterior side gra- 

 dually sloping, and terminating in a narrow, subtruncated, rather 

 short beak ; surface with transverse irregular wrinkles. 

 Coal shale at Middleton, near Leeds. 



16. Pachyodon Heyii. PI. XVI. fig. 10. 



Subtriangular, inflated ; anterior side abruptly sloping ; poste- 

 rior side gradually descending, terminating in an oblique sub- 

 truncation and slightly beaked ; hinge line arcuated, basal line 

 very slightly curved ; umbones prominent, but obtuse and quite 

 close at the beaks; surface with many concentric wrinkles; a 

 longitudinal, gradually widening, shallow groove emanates from 

 the umbones and terminates on the basal margin. 



Ironstone shale at Sheden. 



17. Pachyodon agrestis. PI. XVI. fig. 11. 

 Subcompressed, transversely elongated; anterior side much 



rounded, posterior side lengthened and subacute, descending in a 

 nearly parallel line from the umbones, which are very obtuse and 

 remote ; on the posterior side a longitudinal, wide, oblique, shal- 

 low groove takes its rise on the disc and terminates on the basal 

 margin, below which there is a flexure on the edge ; whole surface 

 covered with very coarse transverse wrinkles ; thickness |ths of an 

 inch. 



Ironstone shale near Sheden. 



18. Pachyodon similis. PI. XVI. fig. 12. 

 Compressed; anterior side rounded from the umbones, which 



are hardly produced but very contiguous ; posterior side nearly 

 Ann. ^ Mag. N. Hist. Vol. xii. 2 E 



