OF CONCHOLOGY. 189 



the extremity ; umbonal slope very oblique, straight, carinated, 

 margined by a wide angular depression ; posterior side flattened ; 

 dorsal margins oblique, straight; lunule profoundly elongated, 

 impressed, with angular margins, but not defined by an impressed 

 line ; ventral margin equally rounded, and the extremities much 

 above the line of the base ; hinge teeth compressed, elevated ; 

 fosset large, oblique ; pallial sinus acutely rounded. 



Dimensions. — Length 4f inches; height 3 inches. 



Locality. — York town, Virg. 



Observations. — This species has been considered a variety of 

 ^S*. delumhis, Con., from the same locality, but it is compressed, 

 whilst the latter is ventricose, and its rounded ventral margin, 

 straight posterior dorsal line, narrower umbo, &c., give it a dif- 

 ferent external character ; the hinge plate is broader, thinner, 

 and the posterior tooth longer than in >S'. dehimhis. It has, per- 

 haps, the most graceful form to be found in the family. 



List of Miocene Species of Hemimactra, Swains on. 



H. congesta, Conrad. IT. subjiariUs, Conrad. 



IT. confragosa, " H. onodicella, " 



If. subcuneata, " S. medialis, " 



PECTINIDiE. 



PECTEN, Lin. 



P. TRICARINATUS, Conrad. 



Description. — Right valve slightly ventricose ; left valve 

 slightly convex, posterior ear sinus not profound ; ribs 15, 

 narrow, square, elevated, each with three prominent equal scaly 

 lines ; interstices much wider than the ribs, with six or seven 

 minutely scaly lines, the middle one generally largest ; in the 

 left valve, the three lines on the ribs have an intermediate line 

 and the ribs are rather wider. Height 3f inches ; length the 

 same. 



Locality. — Virginia. Miocene. 

 Pecten yorkensis, Conrad. 



Description. — Orbicular, thin, depressed ; umbonal region 

 convex ; ribs 21, subquadrate, little prominent ; interstices 

 about as wide as the ribs, with transverse wrinkles, which are- 

 obsolete on the ribs. Height If inches ; length 1^ inches. 



Locality. — Yorktown, Virginia. Miocene. 



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