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ARC A (STErOAIWA) MODIOLiilM n. 8 p. 



Desoription .- Shell small, thin, modioli form, somewhat oom- 

 preascd, very inoquilatei\9.1, nroduoed and higher posteriorly; post- 

 erior dorsal margin direct, long, anterior end rounded "below, sub- 

 trimcate above; rounded posteriorly, the curve being lees convex 

 above; ventral margin sloping downward posteriorly, slightly con- 

 tracted medially; umbones very low, almost terminal; posterior 

 rids-e not well defined distally, but high and broad in the umbonal 

 region; sculoture of low radials, fine and separated by impressed 

 lines in the region before the posterior ria^e, coarser, swollen 

 and separated by as wit'^e or -rider interspaces behind the posterior 

 rido-e; concentric sculpture of fine concentric impressed lines, as 

 well as irree-ular incremental lines, producing a reticulated sculp- 

 ture on unworn portion of the s ell; cardinal area very narrow, en- 

 tirely opisthodetic, deeply impressed, with a low median ridge e:-?:- 

 tending from the anterior end of the area to its ventral nerp-in at 

 about the middle of the posterior series of teeth; anterior teeth 

 numbering four to six, slightly oblique, more or less irregular, 

 subumbonal in position, medial teeth very short and weak, or ob- 

 solete, producing a long edentulous gep; about eight obliqne teeth 

 at the distal end of the posterior series; inner margin of the valve 

 smooth or minutely crenulateci. 



JiT.ensions .- length, 10 mm.; alt. in umbonal region, 4.1 mm.; 

 alt. in posterior region, 6 mm.; semidiam., 1.9 mm. 



