178 CALIFOKNIA Al'AUKMY OK SCIKNCKS. 



The above lU'fJcriptiuii was taken t'ruiu (ioiiKl. A .small fnij^iuent of a shell 

 showing the inuhihition of this species was foiiiul in the lower San Pedro series of 

 Deadaian Ishuul. This fragment may have heen of a young of Panopea generosa, 

 whicli is nnchilated in much the same way as littta. Cooper likens this species to 

 Conrad's Lntfdtht trditsnionlinui from the "Pliocene of T^os Angeles County." One 

 perfect valve from the I'leistoeene at Spanish I'ight, San l)ieg(j. There is no donht 

 as to the identity of the Spanish Bight specimen. 



Living. — San Pedro to Lower California (Carpenter). 



Pleistocene. — San Pedro? (Arnold): San Diego (Arnold). 



Subfamily LUTliARIlN.E. 



Genus Tresus Gray. 



Shell large, inequilateral, thin, inflated; siphonal gape very large, pedal gape narrow; 

 ligament minutely sagittate, separated by a shelly lamina from the pit, which lamina is often recurved 

 and patulous; left cardinal high, compressed; laterals small but distinct in both valves; right 

 cardinal feeble. 



Type, Tresus nuttalli Conrad. 



130. Tresus nuttalli Conrad. 



Lulraria( Crypiodon) niitlal/i Co^.,]om. Phil. Acad. Nat. Sci., Vol. VII, 1837, p. 225, PI. XVIlI.fig i. 



ScAiso/Zia-rus nuttalli Con., Pac. R. R. Rep., Vol. V, p. 234, PI. IV, fig. 33, 1853. Cpr., Brit. 

 Assn. Rept., 1863, p. 637, ^ Luiraria maxima Midd., ^= Mactra ?naxinta (M\dd.) 

 RVE., = Lutraria capax Gld., = Tresus maximus (Midd.) H. & A. Ad.\ms {fide 

 Gahb, Pal. Cal., Vol. II, p. 91, 1869). Tryon, Syst. Conch., Vol. Ill, p. 161, PI. CX, 

 fig. 21, 1884. Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 264. Keep, West 

 Coast vShells, p. 205, 1892. Williamson, Proc. U. S. Nat. Mus., Vol. XV, 1892, p. 183. 



Tresus nuttalli Con., Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 4, 1898, p. 885. 



Shell large, subelliptical, convex, equivalve, inequilateral, thin; umbones small, anterior to 

 center; anterior dorsal margin evenly arcuate, sloping to anterior extremity, which is furthest pro- 

 duced below the middle; posterior aperture truncated, gaping; surface sculptured by prominent lines 

 of growth; one small cardinal tooth in each valve; cartilage-pit large, deep, triangular, separated 

 from ligamental groove by ridge; pallial sinus very large, broad, reaching nearly to line peqien- 

 dicular to beak. 



Dimensions . — Long. 103 mm.; alt. 73 mm.; diam. 44 mm.; umbo to anterior extremity 

 35 mm.; to posterior extremity 68 mm. 



Externally the young of this species resemble .somewhat the young of 

 Saxidomus, but may be distinguished by the thinner, slightly gaping shell, aiul 

 prominent cartilage-pit. Specimens identified by Dr. Dall. 



Common in tlio upper San Pedro series at San Pedro, Los Cerritos, Crawfish 

 George's, and Deadmau Island. Found also in the Plei-stocene at Spanish Bight, 

 San Diego. 



Living. — Alaska to San Diego (Cooper): both sides of North Pacific (Dall). 



