ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY OP SAN PEDRO. 165 



1 10. Macoma yoldiformis Carpenter. 



Plate XVI, Fio. 6. 



Macoma yoldiformis Cpr., Brit. Assn. Rept. 1863, p. 639. Cooper, 7th Ann. Rept. Cal. St. Min., 

 188S, p. 249. Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 5, 1900, p. 1053. 



Shell small, subelliptical, compressed, very thin and fragile; umbones slightly posterior to 

 center; anterior end evenly rounded, longer than posterior side, which is very faintly folded, biangular 

 and more cuneiform; surface smooth, e.xcept for very fine incremental lines; ligamental area scooped 

 out about one-half length of posterior end; teeth very small. 



Dimensions. — Long. 23 mm.; alt. 13.2 mm.; diam. 6 mm.; umbo to anterior extremity 

 13 mm.; to posterior extremity 10 mm. 



This species is distiiigiiisliable froin other members of the genus by its small 

 size, subelliptical form, very thin, smooth .shell, posterior umbones and faint fold; 

 distinguishable from Tellina hodegaisis by less cuneiform posterior end and smaller 

 size. Specimens identified by Dr. Dall. 



Rare in the lower San Pedro series at Deadman Island. Rather common in 

 the Plei-stocene at Spanish Bight, San Diego. The specimen figured is from this 

 locality, and is now in the collection of Delos Arnold. 



Living. — Straits of Fuca to San Pedro (Cooper). 



Pleistocene. — Santa Barbara fCoojJer): San Pedro; San Diego (Arnold). 



Family XXXI. SEMELID^. 



Genus Semele Schumacher. 



Shell rounded, subequilateral, beaks turned forwards; posterior side slightly folded; hinge 

 teeth two in right valve and two in left; laterals elongated, distinct in the right valve; external 

 ligament short, cartilage internal, long, oblique; pallial sinus deep, rounded. 



Type, Tellina reticulata Spengler. 



HI. Semele decisa Conrad. 



Amphidesma decisa Con., Jour. Phil. Acad. Sci., Vol. VII, 1837, p. 234, PI. XIX, fig. 2. 



^(fWif/? rfirma Con. , Carpenter, Proc. Zool. Soc, 1856, p. 213. Carpenter, Brit. Assn. Rept., 



1863, p. 640. Gabb, Pal. Cal., Vol. II, p. 94, i86g. Cooper, 7th Ann. Rept. Cal. St. 



Min., 1888, p. 264. Keep, West Coast Shells, p. 190, 1892. Williamson, Proc. 



U. S. Nat. Mus., Vol. XV, 1892, p. i86. Dall, Trans. Wagner Inst. .Sci., Vol. Ill, 



Part 5, 1900, p. 995. 



Shell rounded, subequilateral, thick; beaks elevated, turned forward; posterior side shorter 

 than the anterior, direct, biangulated; fold distinct; anterior end evenly rounded; anterior dorsal 

 margin slightly depressed; surface with numerous, thick, concentric, rugose, slightly prominent 

 ribs; cartilage-process oblique; cardinal teeth obsolete; lateral teeth distinct; pallial sinus wide 

 and shallow. 



Dimensions. — Long. 8.5 mm.; alt. 78 mm.; diam. 34 mm. 



The specimen described is a large one. Tliese shells are often found in 

 pairs in these deposits. Specimens identified by Dr. Dall. 



