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University of California Publications in Geology [Vol. 9 



MACTRA CALIFORNICA Conrad 

 Plate 16, figures 4a, 46 and 4c 

 Mactra ealifornica Conrad, Jour. Acad. Sci., Philadelphia, vol. 7, p. 240, 

 pi. 18, fig. 12, 1837; Dall, Nautilus, vol. 8, p. 40, 1894; Dall, Trans. Wagner 

 Inst. Sci., vol. 3, pt. 4, p. 876, 1898; Arnold, Mem. Cal. Acad. Sci., vol. 3, 

 p. 174, pi. 19, fig. 2, 1903. 



Mactra augusta Deshayes, Proc. Zool. Soc, London, 1854, p. 67. 



This species was described by Conrad as follows : 



Shell triangular, compressed, equilateral; posterior margin rectilinear; 

 posterior side with a carinated, rectilinear, submarginal line; beaks promi- 

 nent, sulcated; epidermis brown, with concentric wrinkles; teeth lamellar, 

 large, very prominent. 



The hinge plate of this species is broad and flat; chondophore 

 shallow, apically roofed, triangular in shape, overhung but slightly 

 by the arm of the cardinal ; left cardinal prominent, high, thin, 

 without accessories ; anterior arm longer than the posterior, neither 

 reaching to the ventral margin of the hinge plate ; right cardinal 

 also high, thin, arms fused along the dorsal side of the tooth ; 

 anterior arm makes a relatively wide angle with the anterior dorsal 

 margin of the shell ; laminae smooth ; anterior lateral bifid, the 

 dorsal portion being usually the thinner and possessing the char- 

 acter of a lamella ; this tooth is short and high, originating as a low 

 ridge dorsal to the middle of the anterior cardinal arm and extend- 

 ing parallel to the ventral margin of the hinge plate ; anterior 

 ventral lamina very small, inserted upon the dorsal margin of the 

 shell ; posterior laminae short, the small dorsal one arising from the 

 shell margin. Pallial line distant, with a deep and rounded sinus 

 which reaches nearly to the middle of the shell and joins conflu- 

 ently for a short distance with the pallial line. The measurements 

 of the type as inferred from the original cut are : length 36 mm., 

 height 23 mm. 



This species has been confused with Spisula catilliformis, the 

 latter having been listed in much of the literature under the name 

 of the former. Specimens of these two species are easily distin- 

 guishable. M. ealifornica resembles S. falcata in outline, but it is 

 separable from that species upon the basis of its sulcate beaks, and 

 prominent cardinal with its widely diverging arms. It differs from 

 M. nasuta in being more ventricose, and in possessing longer lat- 

 erals. 



