of Burlington, loiuu, and its Vicinity. 231 



space between them less than their width. The implanted 

 costae increase very gradually, and sometimes irregularly, 

 in size. The whole crossed by very fine, undulating, con- 

 centric striae. On a narrow space along the cardinal 

 border of the ventral valve the bases of numerous small, 

 thickly-set spines are observed, but no indication of spines 

 has been detected on other parts of the shell. 



This species somewhat resembles the shell in the Kas- 

 kaskia limestone, usually referred to P. Cora, but differs 

 from it in its less prominent and broader beak. The 

 costsB are also coarser, smoother, and more rounded and 

 elevated. 



In consequence of the compactness of the imbedding 

 stone, specimens are rarely found showing the full length 

 of the hinge-line, or the expansion of the front. 



Geological formation and locality. In beds Nos. 1, 5, 6, 

 and the lower portion of No. 7, of the preceding sections, 

 Burlington, Iowa. 



ORTHIS THIE3IEI (White.) 



Shell depressed, orbicular, usually a little wider than 

 long, widest below the middle, hinge-line very short. 

 Dorsal valve deeper than the ventral valve, regularly con- 

 vex, with the general exception of a very shallow sinus 

 extending from beak to front. Beak projecting a little 

 beyond the hinge-line, and slightly curving toward the 

 beak of the opposite valve. Cardinal process strong, with 

 a strong, rounded septum extending from it nearly half the 

 length of the shell. Brachial processes strong, notched at 

 the ends. Margins crenulate more than half around the 

 shell. 



Ventral valve convex near the umbo, depressed in front, 

 which, with the depression on the opposite valve, consider- 

 ably flattens the front margin. Beak short, elevated, and 



