Capt.T. Brown on some new species of the genus Pachyodon. 391 



cated ; hinge line almost parallel ; external surface with a few 

 remote concentric wrinkles or lines of growth. 



I found this species in the coal shale at Dalkeith, Mid Lothian. 



2. Pachyodon lateralis. PI. XV, fig. 3. 



Transversely elongated, subquadrate cuneiform ; sides very 

 unequal, the anterior one very long, gradually sloping from the 

 umbones and terminating in an obliquely truncate point ; pos- 

 terior one very short ; umbones produced, with acute but not in- 

 flected beaks. Length somewhat more than half an inch ; breadth 

 nearly an inch and a half. 



Coal shale, Whitehaven. 



3. Pachyodon sulcatus. PI. XV. figs. 4, 5. 



Subtriangular, rather compressed; umbones prominent, very 

 close, slightly reflected, subacute, and placed considerably to one 

 side ; general surface smooth, with inequidistant concentric fur- 

 rows ; posterior side arcuated, with a rounded point situate low ; 

 anterior side gently rounded ; basal line nearly parallel. Length 

 \^ inch; thickness ^ inch. 



Coal shale near Whitehaven. 



This species is liable to some variety in external contour. 



4. Pachyodon rugosas. PI. XV. figs. 6, 7. 

 Subtriangular, greatly ventricose in proportion to its size, its 



depth being equal to five-sixths of its whole length; umbones 

 very prominent, situate considerably to one side, pointing poste- 

 riorly and remote from each other; anterior side abruptly de- 

 scending and rounded ; posterior side gradually sloping and con- 

 siderably more acute than the other ; ligament produced ; exter- 

 nal surface with unequal, rugose, concentric wrinkles. Length 

 2|^ inches ; breadth 3^ inches ; thickness 2^ inches. 



The young shells are much more rugosely wrinkled than the 

 adult. 



Pound in the ironstone shale at Sheden by Mr. S. Gibson of 

 Hebden Bridge, and in his cabinet. 



5. Pachyodon suhrotundus. PI. XV. fig. 8. 



Subrotund ; umbones subcentral, produced, blunt, and some- 

 what remote from each other ; hinge line considerably arcuated ; 

 surface with irregular, acute, concentric wrinkles; thickness 

 about equal to half its length. 



Coal shale, Oldham. 



6. Pachyodon bipennis. PL XV. fig. 9. 

 Transversely elongated, somewhat hatchet-shaped; sides un- 

 equal ; umbones produced and remote ; hinge and basal lines 



