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29 <br />There appears to be some correlation, albeit informal, between <br />the overall shape of a mano and the type and degree of intentional <br />modification present. hlost are oval in outline, grading almost to <br />a rectangle when heavy pecking, use, and grinding of nonworking sur- <br />faces is present. With less modification, shapes tend to be more ir- <br />regular; unmodified cobbles retain much of their original shape. <br />~ Where these are modified, it is restricted to pecking on the grind- <br />ing surface alone. This will either flatten it and thereby increase <br />surface contact area with the metate, or roughen the grinding sur- <br />face to increase grinding efficiency. <br />Ground stone: unprepared bifacial mano, oval shape, Type IIA. Two Specimens. <br />Description: generalized oval outline, either biconvex or plano- <br />convex cross section. There is no visible alteration of natural <br />cobble other than use. <br />Material: sandstone. <br />Size range: thickness, 40 mm. <br />Special notes: in piano-convex specimens the flat grinding face <br />usually has better polish than the convex surface. <br />Ground stone: angular bifacial mano, Type IISA. One SpeClmen. <br />Description: polygonal shape, piano-convex cross section. <br />Material: basalt. <br />Size range: thickness, 60 mm. <br />Special notes: better polish on the flat face. The convex surface <br />is smoother than natural but has no polish. <br />Ground stone: prepared unifacial mano, Type IVA. Three specimens (Fig. 12b). <br />Uescription: there is a grinding facet on one side only. The com- <br />plete specimen has an oval outline. The thickness is the least <br />dimension. <br />Materials: sandstone and quartzite. <br />Edge wear: pecking on margin and grinding surface. <br />Variation: one specimen has margin preparation only. The others <br />are pecked on the margin and the grinding surface. <br />Size range: length, 90 mm.; width, 74 mm.; and thickness, 38 mm. <br />Special notes: one specimen is fractured lengthwise on a horizontal <br />plane, and thus has only one face. It is included in this catagory, <br />but could just as easily be bifacial. <br />Ground stone: repared, oval, bifacial mano, Type IvB. Seven specimens <br />ig. b . <br />Description: pronounced pecking on both grinding faces, margins, and <br /> <br /> <br />