- 416 - 



cariub it. sharper on right v.ivt'a than on left Oi.eb; ox: l^ulis 

 sitbII left valves, however, it is pronoiiiieecHy angular and the 

 C'^rinal rilD maj be slightly exaggerate'' anrl such :^ . ::;pi-oach 

 tlie variet:" xa:,T.iacenae . althoun-h they retain the orincipal char- 

 acters .of the tj'pieal form. The numher of rihs santerior to the 

 carina r.iaj' he as low as twelve, but thirteen or fourteen is 

 usual; behind the carina nine is the predominant nuiiiber, although 

 eig'ht or ten may be present. The nodules raay be absent from son© 

 or all of the ribs; in fact, no rjin?-le specimen h-^s all of them 

 intacc. ..hei. deprived o' their nodules the ribe are, sraooth and 

 polished, but have suggestions of incremental lines. sur- 



face of the ribs or interG"icces is remove'"'', both Eho'v -^ine con- 

 centric striae, finer than those on the unworn inter. . ::^i 

 interesting feature lies in the keeling of the ribs '. i.tely 

 ai-.terior to the crrina. These ribs, althou~h 3.ctuall -."if^e as 

 those proceeding, appear narrov/er, Gin';e the flat-to; 

 corresponds to the entire width of the preceedir.g ribs. The keel 

 decreases in vrl'^th "oostfrrim-j ;;, 'Roth th (- Inr.-rr -inrti on of th^o 

 rib- and the keel it.oelf are usually square. A vory narrov;, G..'.ooth 

 area, more prominent on yomig shells, separates th rior i^or- 



sal margin fr.-'-^ '-"c "irn't rib. ?' ■- ?'' istl.. ' '. ^ :.re of 



0. haitense is tixe sculiotare which consist -jlly twenty-three 



nodulatec", high, square ribs, v/hich are separated by concentrically 

 striated, deep int erspaces -of th'- -■^^■n <--,it>^. Ttp tT -; -hr" iv^-^i on 



