AKNOLD — THE PALEONTOLOGY AND STKA I KiUAni V OF SAN PEUKO. 13o 



Fiimily XXllI. CKYPTODONTID.E. 



Genius Thyasira I.cacli. 



Shell globular, posterior side furrowed or angulated; umbones much iLCiuved; hiiuile sliuil or 

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

 and outside the hinge-plate; teeth altogether wanting. 



2'hijasira Jlexiiosus Mont, is ti characteristic species. 



65. Thyasira bisecta Conrad. 



Platk XV. Fig. 5. 



Venus bisecta Con., Wilkes E.xpl. Exped., Vol. X, App. i, p. 724, PI. X\'ll, ligs. 10, \oa, 1849. 



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



Cyprina bisecta <ZQfiii.,]o\xx. Conch., 1S65, p. 153. 



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



Conchoccle disjiincta Gabb, Pal. Cal., Vol. II, pp. 28, 29, PI. VII, fig. 48, 1869. Cooi'er, 7th Ann. 



Rept. Cal. St. Min., 18S8, p. 236. 

 Cryptodon bisectiis Con., Dall, Proc. U. S. Nat. Mus., Vol. XV, 1892, p. 1S9; 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 directioii ; 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. 



Gabbdescribes this shell as occurring in the Miocene of Deadman Island, wliiU; 

 Cooper gives its occnrrence as in the " Quaternary " ot" the same ]ilace. It is found 

 most abundantly in a hard stratum near the base of the Deadman I.sland Pliocene 

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

 found also at Timm's Point in the Pliocene. The specimen ligured is from the 

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



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



Pliocene. — San Pedro (Arnold). 



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

 (Arnold). 



66. Thyasira gouldi rhilipjii. 



Cryplodo7i flexuosHs (not of Montagu) of Carpenter, Brit. Assn. Rept., 1863, 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. 



