ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 175 



125. Mactra exoleta Gray. 



Plate XIX, Fig. 4. 

 Mactra exoleta Gray, = Lutraria venlricosa Gld. (yft/d? Carpenter, Proc. Zool. S(jc., 1856, p. 200). 



Shell of medium size, decidedly Irigonal, ventricose, thin, fragile; umbones slightly anterior 

 to center, elevated, not touching, turned only slightly forward; anterior dorsal margin straight; 

 anterior e.xtremity evenly rounded near base; ventral margin evenly arcuate; posterior dorsal margin 

 arcuate, making an acute angle with the ventral margin; a sharp, prominent, angular ridge runs from 

 umbo to the posterior extremity; surface sculptured with fine, incremental lines, which are most 

 prominent posterior to ridge; cartilage-pit deep; hinge-teeth lamellar, long; ligamental groove very 

 deep and narrow. 



Dimensions. — Long. 56 mm.; alt. 43 mm.; diam. 30 mm.; umbo to anterior e.xtremity 

 23 mm.; to posterior extremity 33 mm. 



Distinguishable from others of genus by the sharply angular trigonal shape, 

 great convexity, elevatetl umbo, and sharp, submarginal posterior ridge. Specimen 

 identified by Dr. Dall. 



Rare in upper San Pedro series of San Pedro; one nearly perfect right valve, 

 which is figured, and is now in the collection of Delos Arnold. Found also in the 

 Pleistocene at Spanish Bight and Pacific Beach, San Diego. 



Living. — Coast of Mexico (Carpenter). 



Pleistocene. — San Pedro; San Diego (Arnold). 



126. Mactra hemphilli Dall. 



Pl.\te XIX, Fu;. 3. 

 Mactra hefuphilli Dal-v, Nautilus, Vol. VII, 1894, p. 137, PI. V. 



Shell large, subtrigonal, subequilateral, convex, rather thin; umbones slightly elevated, 

 subcentral, turned slightly forward; anterior portion hollowed in front of umbo, making the dorsal 

 margin slightly concave; anterior end evenly rounded; posterior dorsal margin evenly arcuate, 

 making a shorter turn into the ventral margin than does the anterior dorsal margin; posterior end 

 most produced near base; a prominent angular submarginal fold runs from the umbo to the posterior 

 extremity; hinge same as in M. calif arnica, except that the anterior ligamental groove is much shorter; 

 pallial sinus of medium size, very oblique. 



Dimensions.— \^Q\-\^. 123 mm.; alt. 100 mm.; diam. 62 mm.; umbo to anterior extremity 

 50 mm.; to posterior extremity 73 inm. 



This species is distinguishable from M. calif ornica by its greater altitude, more 

 concave anterior dorsal margin, shorter anterior ligamental groove, and oblique pallial 

 sinus; one of its unique characteristics being the oblique sinus. Specimen identified 

 by Dr. Dall. 



Rare in upper San Pedro series of San Pedro; one specimen, which is figured, 

 and is now in the collection of Delos Arnold. 



Pleistocene. — San Pedro (Arnold). 



Subgenus Spisula Gray. 



ate, set in a ca 

 lelly ridge. 



Type, Mactra solida (Linn.) Gray. 



Mactrae with ligament sagittate, set in a callous area close to the dorsal margin, and not set 

 off from the chondrophore by any shelly ridge. 



