a new genus of Fossil Shells. 483 



I think it probable that this species may, upon examination 

 of numerous specimens, prove to be merely a variety of P. Lis- 

 ten. 



3. Pachyodon imbricatus, n. s., Plate IX. fig. 5, 6. 



Shell subtriangular ; lines of growth thin- edged, imbricated and 

 numerous; lunule cordate ; height l'l, length 1*5, thickness 0*7. 

 Lias, banks of the Severn, Gloucestershire, and Bishport, Somer- 

 setshire. 

 Museum of Bristol Institution. 



4. Pachyodon crassissimus, Plate IX. fig. 7. 



Shell subtriangular ; umbo incurved towards the anterior margin *, 

 lines of growth forming deep transverse sulci ; height 2 - 6, length 2*8, 

 thickness 1*6. 



Inferior oolite, Dundry Hill, Somersetshire. 



Unio crassissimus, Sowerby, Min. Conch., tab. 153. 



Museum of Bristol Institution, George Cumberland, Esq. 



This species I have little doubt is the Unio crassissimus of 

 Sowerby, although I do not understand his figure as regards 

 the teeth; the shell is remarkably thick, being 1*6 from the 

 outside extremes of one valve to the other, while the space for 

 the animal is only 0*6. 



I have seen another specimen from Wick, near Bath, three 

 inches in height and nearly four in length ; both in the col- 

 lection of George Cumberland, Esq. 



5. Pachyodon crassiusculus, Plate IX. fig. 8. 



Shell ovate, symmetrical ; lines of growth equidistant ; anterior 

 extremity ^th of the length ; height 1*8, length 2*5, thickness 1*1. 

 Lias, Langar, Nottinghamshire ; near Cheltenham ; Blue Anchor, 



Somersetshire ; Robin Hood's Bay, Yorkshire. 

 Unio crassiusculus, Sowerby, Min. Conch., tab. 185. 

 Pullastra, Phillips, Geol. of Yorkshire, pi. 13. fig. 16. 

 Museum of Bristol Institution, Wm. Bean, Esq., Scarborough. 



It is evident from the figure in the ( Mineral Conchology/ 

 that the Unio crassiusculus belongs to this genus, three or four 

 specimens having come into my possession. I have no doubt 

 of its being a distinct and well-characterized species : it is the 

 most symmetrical of the genus. 



I very strongly suspect that the locality given in the i Mine- 

 ral Conchology ' must be a mistake, originating from the close 

 resemblance in colour which the fossils of the ferruginous 

 marlstones have to the fossils of the crag formation. 



* This portion of the shell is frequently described as the posterior in the 

 ' Mineral Conchology/ and vice versa. 



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