Paleontology Zorritos Formation 165 



Superfamily Mactracea 



Family MACTRIDAE 



' Subfamily Mactrixae 



Genus Mulixia Gray 



Mulinia zorritensis ( X el son) 

 Plate X— Figs. 8. 9. 



Mactra zorritensis Nelson, Trans. Conn. .\cad., vol. 2. p. 201, 

 1870. 

 Mactra sp. ind., Nelson, ibid., p. 201. 1870. 



Shell small, very delicate, inflated, triangular. Anterior 

 margin nearly straight dorsally. curving into the ventral 

 margin, which is elliptical ; posterior margin nearly straight, 

 meeting the ventral margin in a sharp angle. Umbones sub- 

 central in position, apically small, inflated, prosogyrate, 

 about 1.25 mm. distant in a shell 22 mm. long by 19.5 mm. 

 high. Anterior surface evenly convex ; medium surface 

 longitudinally flat and vertically convex. A sharp posterior 

 carina is distinct from the top of the beak to the posterior 

 marginal angle ; behind it the surface is flat ventrally and 

 slightly excavated near the beaks. Sculpture of fine con- 

 centric lines ; in some shells irregular diflFerentiation of 

 growth-stages gives a somewhat rough appearance, and 

 others are more nearly smooth. Dorsal areas distinct, de- 

 limited by sharp lines which form a lanceolate posterior area 

 and an olive-shaped anterior area ; surface of areas finelv 

 striate longitudinally. Hinge normal, very delicate. Di- 

 mensions of 2 specimens: Length. 19.5: height. 16: diame- 

 ter. II mm. Length. 22: height. 195: diameter. 9.5 mm. 



Xelson differentiated the Mulinias in his collection, 

 naming a portion Mactra corritcnsis. and leaving the others. 

 which show all characters very well, as indeterminate. He 

 says concerning his "indeterminate" species, "This species 

 may be told from the preceding '{M. corritensis)' which 

 it much resembles, in being broader, having less prominent 

 umbos, and being less convex ; posterior margin no: so angu- 



