LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

description of species 121
Opi* jftasfaiis Anderson, n, sp.
(PlKo flcum 4, 5)
Shell small, obliquely ovate, cordate; beaks high, rounded, smooth, closely
approximate, curving inward; valves aubtriangular in outline, base broadly rounded;
hinge ehort, teeth not showing; surface smooth, polished, pearly, showing indistinct
growth lines; umbonal ridge oblique to hinge line, nearer to the anterior margin;
lunule small, impressed, not deep, lanceolate; shell flattened slightly behind the
beaks. Two Bingle valves of this shell were obtained, of which the holotype (Calif.
Acad. Sci. type Coll.), measures; length near base, 17 mm.; height, 17 mm.; thickness
of single valve, 0 mm.
Both examples were obtained at Locality 1347 (Calif. Acad. Sci.) on Mitchell
Creek, a few feet above tbe conglomerate of the Barr zone, where they were associated
with Lytccerot (Gabiyiocera&) angulalum and AcroUuihis aboriprnoftV, nov.,
near tho lower limit of the Aptian portion of the Horsetown group.
LBCIXIDAE Fleming
Lucina alcatrazis Anderson, n. sp.
(Plata 5, fl^we 7)
Shell rather large, subcircular in outline, little inflated, ends rounded, curving
regularly to the rounded basal border; posterior dorsal margin broadly rounded,
flattened near the beaks; beaks subcentral, nearer anterior end, approximate, excavated
in front by a email, poorly defined lunule; anterior border sloping steeply,
rounded below; surface marked by fi-ne concentric lines over most of it, and by welldeveloped
posterior plications extending from behind the beaks to tbe sharply bent
postero-ventral margin. The interior features of the shell, as shown in one example,
are lueinoid, and include well-developed lateral teeth and two cardinals in the
left valve, between which there ia a triangular pit; tbe anterior adductor muscle
is rather long, extending below the median line of the valve. The interior of tbe
valve is rough and irregular, with faint concentric undulations. The holotype of
this species is a well-preserved rock mold, from which good casts can readily be
made, showing the external features of the shell. It has the following dimensions:
length, 53 mm.; height, 41 mm.; thickness of single valve, 9 mm. Another example
of the same species shows the hinge, anterior adductor muscle scar, and the interior
form of the shell. Three examples of thia species, including tbe holotype, were
obtained by Gabb from Aleatraz I aland, Bay of San Francisco, and were among the
"Numerous casts" of the "other bivalves" mentioned by him in hie description of
Inoctramua cllioli (1809a, p. 193), which came from the same place. These examples,
apparently labelled by Gabb, were left with other material from the same place at
the University of California.
IBOCAJUHIDAE Gray
Clitocolvs Gabb
CUwcolw indubiius Anderson, n. sp.
Shell small, circular in outline, moderately inflated; beaks sub-central, elevated,
curving forward, approximate; hinge margin gently curved; external ligamental
scar narrow; lower border of shell nearly circular, thin, not crenulated; surface
smooth above, marked by concentric lines below, which become irregularly spaced
near the lower margin.
This shell somewhat resembles Clixocolux dubimt (Gabb) from the Upper Cre-