PALEONTOLOGY ZORRITOS FORMATION 95 



Superfamily ARCACEA 



Family ARCIDAE 



Subfamily ARCINAK 



Genus ARCA (Linnaeus) Lamarck 



Subgenus NOETIA Gray 



Area (Noetia) cholana n. sp. 



PLATE V Figs. 2, 3. 



Shell triangular in general outline, moderately inflated. 

 Anterior margin rounded, merging into the upper margin of 

 the ligament area ; ventral margin slightly rounded, posterior 

 margin unknown. Umbones opisthogyrate, situated just for- 

 ward of the middle, flattened above, the upper surface sloping 

 forward. Umbonal slope gently rounded anteriorly, the gen- 

 eral anterior surface being only slightly convex, and the 

 central surface similar ; posterior keel very sharp and distinct 

 from the top of the beak to the ventral margin, curving out- 

 ward ; posterior slope slightly excavated. Ribs, 35 in num- 

 ber, flat and low, with many fine, wavy, delicate, imbricated 

 growth-lines which cross both ribs and interspaces. Ribs 

 about equal in width to interspaces. A fine subsidiary rib is 

 present in the posterior interspaces, its strength increasing 

 posteriorly. Area in two parts; the ligamental part, before 

 the beaks, bearing pronounced transverse striations and 

 being hollowed out to form part of a cylinder. Just beneath 

 the beak the transverse striations change suddenly, becoming 

 very fine and numerous for a distance of about 2.5 mm. 

 The ligament area terminates directly below the point of 

 the beak, behind which is a lunule of roughly triangular 

 shape, the longer upper limb of the triangle being the last 

 posterior rib; it is crossed transversely by a distinct V- 

 shaped sulcus, and longitudinally by very fine parallel stria- 

 tions. The hinge-plate is moderately wide posteriorly, bear- 

 ing medium-coarse teeth, which merge into the transverse 



