10 PLATE V. [Assembly 



Fossils of the Medina sandstone. 



A. LiNGFLA CUNEATA : Three specimens, as found on the surface of 

 a layer. Ripplemarks are visible on the rock ; and a little ridge 

 of hardened sand extends from each shell, doubtless formed- by 

 the tide-current when the shells lay on the sandy sea-bottom, 

 now hardened to rock ( See Hall's Report, 3d Dist., p. 52). 



1. Pleurotomaria parvetusta, base. 



2. Same, side view. 



3. MODIOLOPSIS PRIMIGENIUS. 



4. DiSCINA PARMULATA. 



5. LiNGULA CUNEATA. 



6. BUCANIA TRILOBATUS, back. 



7. Same, profile. « 



8. MODIOLOPSlS ORTHONOTA, slde. 



9. Same, back. 



10 & 11. Arthrophycus harlani ( p. 52). 



