ARNOLD — THE PALEONTOLOGY AND STKATIGRAPHY OF SAN PEDKO. 135 



Family XXIII. CRYPTODONTID^E. 



Genus Thyasira Leach. 



Shell globular, posterior side furrowed or angulated; umbones much recurved; lunule short or 

 indistinct; ligament usually and to a certain extent external, placed in a groove on the hinge-line, 

 and outside the hinge-plate; teeth altogether wanting. 



llujaslra Jiexuosus Mont, is ;i characteristic species. 



65. Thyasira bisecta Conrad. 



Platf, XV. Fig. 5. 



Venus bisecta Con., Wilkes Expl. Exped., Vol. X, App. i, p. 724, PI. XVII, figs. 10, loa, 1849. 



Thyatira? bisecta Con., Meek, Smithsonian Inst., Check-List Mioc. Fossils, 1864. 



Cyprina bisecta Co*i., Jour. Conch., 1S65, p. 153. 



Conchocele bisecta CoN. Gabb., Pal. Cal., Vol. II, p. 99, 1869. 



Coyichoccle disjuncta Gabb, Pal. Cal., Vol. II, pp. 28, 29, PI. VII, fig. 48, 1869. Cooper, 7th Ann. 



Rept. Cal. St. Min., 1888, p. 236. 

 Cryptodon bisectus Con., Dall, Proc. U. S. Nat. Mus., Vol. XV, 1892, p. 189; Vol. XVII, 1894, P- 



713, PI. XXVI, figs. 2-5. 



Shell large, subquadrate, convex, of medium thickness, equivalve, inequilateral ; beaks 

 terminal, anterior; anterior end abruptly and angularly truncated; posterior dorsal margin arcuate, 

 sloping down toward posterior extremity; surface marked only by lines of growth except near the 

 posterior part, where the peculiar truncation takes place, the surface suddenly descending at right 

 angles to the curve of the shell, for a short distance, and then resuming its former direction ; ligament 

 long and elliptical; large lunular area in front of the beaks. 



Dimensions. — Long. 72 mm.; alt. 62 mm.; diam. 41 mm. 



Specimens identified by Dr. Dall. 



Gabb describes this shell as occurring in the Miocene of Deadmau Island, while 

 Cooper gives its occurrence as in the " Quaternary " of the same place. It is found 

 most abundantly in a hard stratum near the base of the Deadraan Island Pliocene 

 deposits; a few shells, however, have been found in Pliocene strata above this layer; 

 found also at Timm's Point in the Pliocene. Tlie specimen figured is from the 

 Pliocene of Deadman Island, and is now in the collection of Delos Arnold. 



Living. — Paget Sound, sixty-nine fathoms (Dall). 



Pliocene. — San Pedro (Arnold). 



Miocene. — Astoria, Oregon (Conrad): Blakely, near Seattle, Washington 

 (Arnold). 



66. Thyasira gouldi Philippi. 



Cryptodon flexuosHs (not of Montagu) of Carpenter,- Brit. Assn. Rept., 1S63, p. 643. Cooper, 7th 

 Ann. Rept. Cal. St. Min., 1888, p. 237. 



Shell small, globular, posterior side angulated or furrowed ; umbones much recurved ; sur- 

 face sculptured with fine incremental lines ; lunule indistinct, depressed in front of beaks ; ligament 

 external, placed in a groove on the hinge-line and outside the hinge-plate ; teeth wanting. 



Dimensions. — Long. 9.5 mm.; alt. 11 mm.; diam. 8 mm. 



