X78 CALIFOENIA ACADEMY OF SCIENCES. 



The above description was taken from Gould. A small fragment of a shell 

 showing the undulation of this species was found in the lower San Pedro series of 

 Deadraan Island. This fragment may have been of a young of Panopea generosa, 

 which is undulated in much the same way as Bcbta. Cooper likens this species to 

 Conrad's Lutraria transmontana from the " Pliocene of Los Angeles County." One 

 perfect valve from the Pleistocene at Spanish Bight, San Diego. There is no doubt 

 as to the identity of the Spanish Bight specimen. 



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



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



Subfamily LUTRARllN.^. 



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 nuttalll Conrad. 



130. Tresus nuttalli Conrad. 



Lntraria( Cryptodon) w?<//a///CoN., Jour. Phil. Acad. Nat. Sci., Vol. VII, 1837, p. 225, PI. XVIII, fig i. 



Schizothccrus nuttalli CoN. , Pac. R. R. Rep., Vol. V, p. 234, PI. IV, fig. 33, 1853. CPR. , Brit. 

 Assn. Rept., 1863, p. 637, ^= Lutraria ^naxiyna MiDD., = Mactra maxima (}>i\v>v>.^ 

 RvE. , = Ltitraria capax Gld. , ^ Tresus maximus (Midd.) H. & A. Adams {,f-de 

 Gabb, 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 Shells, 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 perpen- 

 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, and 

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



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

 George's, and Deadman Island. Found also in the Pleistocene at Spanish Bight, 

 San Diego. 



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



