PALEONTOLOGY ZORRITOS FORMATION 103 



125. Umbones prominent, full, incline forward, strongly 

 prosogyrate, situated at the anterior third of the entire 

 length. General surface convex; posterior slope slightly 

 excavated behind a broad keel which descends from the 

 umbone to the marginal angle. Sculpture of 39 low, flat- 

 topped ribs, of which the anterior fourteen are longitudinally 

 sulcate near the margin, and the remainder are smooth. 

 Interspaces, evenly equal in width to the ribs, show concen- 

 tric lines which are less prominent to obsolete on the ribs. 

 Area extremely narrow, nearly three-fourths of its length 

 being behind the middle of the beaks, separated from the 

 rest of the sculpture by a raised ridge. Teeth fine, distally 

 oblique. Inner shell unknown. Length, 29.5 ; height, 23.5 ; 

 (semi) diam., 10.5; length of hinge, 18.5 mm. 



The form which seems most closely related to Area fissi- 

 costa is A. hypomela Ball, 100 from the upper Oligocene of 

 Ballast Point, Alum Bluff, and the Chipola marl of Florida. 

 It differs in being much more produced posteriorly, and thus 

 longer in proportion to height, and in having the ribs chan- 

 neled over a larger part of the shell. In number and type 

 of ribs, and in general appearance the two are clearly similar. 

 In the local fauna A. singewaldi var. do ma is somewhat 

 similar, but rarely reaches the size of A. fissicosta, and in 

 addition has nine to ten less ribs. 



Lower Zorritos. Quebrada Zapotal. 



Area (Scctpharca) singewaldi n. sp. 

 PLATE V Figs. 12, 13. 



Shell small, inflated, inequilateral, oblique. Anterior 

 margin rounded, somewhat truncate, meeting hinge-line at 

 an angle clearly greater than 90, but not markedly obtuse; 

 ventral margin evenly elliptical ; posterior margin almost 

 angular, meeting hinge-line at an angle of about 140, and 

 forming a straight line between the hinge and the end of 



Idem., p. 637, pi. 33, fig. I. 



