- If 



Section Ouiiearca Dall. 

 AROA (OUITEARGA) OPTHAIJTA n. sp. 



De script Ion .- Shell small, cardiforni, hei.p'ht esceeclinp- the 

 length, decidedly inflated, higher posteriorly, moderately inequi- 

 lateral; left "valve larsrer, higher posteriorly, more inflated than 

 the r i^-ht and having higher mnhones; riorsal margins short; rounded 

 anteriorly and posteriorly, the curve of the anterior margin short- 

 er and more convex than the posterior, passing mthoiit interru-ntion 

 into the slightly curved hase; hase descending tovi^ard the posterior 

 end; umhones high, tumid, subcentral in position, prosog^rate; ex- 

 ternal surface sculpti^red v/ith twenty-six to twenty-eight radial 

 ribs, conspicuously nodulated throughout on the left valve, ex- 

 cept on the umbo, but the right iralve haviiig- only a few anterior 

 ribs nodulated; cardinal area narrov/ behind the umbones, shorter 

 and wider before; anterior series of teeth numbering eight or nine, 

 vertical, forming an uninterrupted series, with the tv/elve to four- 

 teen teeth of the posterior series; inner margin of the valve 

 with short, deep crenulations . 



Dir:iensions .- length, 7.5 ::r . ; alt. 7.8 mm.; semidiam., 

 3.5 mm.; length of hinge, 4.5 mm. 



Remarks .- It is not i-nreasonable to assume that the sr:Hll 

 valves that are fre only re-oresentatives of this form a.re immature. 



