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



parallel, obliquely truncate, with a slightly turned- up beak below ; 

 hinge line nearly straight, basal line somewhat arcuated ; surface 

 irregularly wrinkled transversely. 

 Coal shale at Middleton, near Leeds. 



19. Pachyodon turgidus. PI. XVI. figs. 13, 14. 



Inflated ; thickness nearly seven-eighths of an inch ; breadth 

 an inch and three-eighths ; umbones prominent, set a little apart ; 

 anterior side short, slightly subtruncate ; posterior side nearly 

 parallel above, with a truncated termination ; hinge line almost 

 parallel, basal line with a slight flexure; surface with pretty 

 strong irregular wrinkles. 



Coal shale at Wakefield. 



20. Pachyodon nucleus. PI. XVI.* fig. 1. 



Inflated, transversely ovate; umbones subacute and remote; 

 hinge line nearly straight ; anterior side a little acute, posterior 

 side elongated and acuminate; basal line subarcuated; surface 

 with shallow transverse wrinkles. 



Coal shale at WoodhaU, on the north side of the Pentland Hills, 

 near Edinburgh. 



21. Pachyodon Blaydsii. PI. XVI.* fig. 2. 



Obhquely subtriangular, inflated; umbones prominent and 

 remote ; hinge line nearly straight ; anterior side parallel above 

 its termination, suddenly rounded; posterior side acuminated, 

 straight above, with an obliquely truncated termination, sharply 

 beaked below ; basal line ascending from the point opposite the 

 umbones. Length fths of an inch; breadth Jths; thickness 

 nearly half an inch. 



Coal shale at Middleton, near Leeds. 



22. Pachyodon Aldamii. PI. XVI.* fig. 3. 



Subcompressed, flexuose and subtriangular; umbones sub- 

 central, very obtuse, set ^th of an inch apart ; hinge line slightly 

 arcuated; anterior side abruptly descending from the umbones, 

 beneath which it is slightly rounded with a flexure below, some- 

 what produced on the margin immediately under the umbones ; 

 posterior side gently sloping and rounded, with a shallow furrow 

 emanating from below the umbones, and rapidly widening, termi- 

 nates on the base; basal line flexuose. Length 1| inch; breadth 

 2 J inches ; thickness 1 inch. 



The greatest thickness of the shell is at the middle of the discs, 

 from whence it rapidly becomes thin towards the margins. 



Coal shale at Whitehaven. 



23. Pachyodon antiquus. PI. XVL* fig. 4. 

 Transversely elongated, subcompressed; umbones very obtuse 



