LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

description of species 147
it differs from it notably, having a section higher than broad, which is the reverse
of the other; there are greater differences in sculpture. In its younger stages the
shell of L, aulatum is often smooth, but this condition is soon lost. When the coatae
begin to appear they are leas regularly spaced but with growth they become more uniform.
The interspaces are at first relatively broad but later becomo occupied by
secondary costal lines. In mature shells the costae become more crowded, costal
lines appearing even on the slopes of the primary coatal ridges. In young stages the
costal lines in the ventral zone are crossed by fine revolving lines parallel to the
siphonal plane. In older specimens these revolving linea spread downward on the
sides of the shell. In more mature stages these intersections of the costal lines break
up, and the costal ridges become crenulatcd, producing on tbe surface of the shell a
reticulated lace-like pattern, as shown in the figure. The crenulations form points
on the posterior side of the coBtal ridge, leaving the anterior aide looped or nearly
smooth. In its sculptural feature* the shell resembles Amm. eubfimbrialus d'Qrbigny,
from the Neocomian of France, although the castas are much less crowded,
and there are other obvious differences. The holotype (Calif. Acad. Sci. type Coll.)
is a nearly complete epecimen, having a portion of the shell preserved. It was
obtained at Locality 113 (Calif. Acad. Sci.) in the Hamlin-Broad zone at the head
of the west branch cf Mitchell Creek, 4 miles southwest of Ono, Shasta County. It
was found associated with species of Polyptychitea and of jStm&tYsfcitei. Specimens
of the same species were found on Fiddler Creek, a mile ahove its mouth beneath a
thick bed of conglomerate which seems to mark tbe base of the Horsetown group.
Fragments of the same species have been found in the lower beds of the Horsetown
but for the most part it seems to character! ze the upper part of the Paskenta group
in the Cottonwood district; it has not yet been found in the district south of Elder
Creek. Measurements on the holotype gave the following dimensions: greatest
diameter, 237.6 mm.; width of umbilicus, 118.7 mm.; heigh t of wborl, 68.5 mm.; width
of whorl, 61.5 mm.; umbilical ratio, 0.5:1.
The horizon of the holotype is in the upper beds of the Paskenta group, about 500
feet beneath the conglomerates that mark the base of the Horsetown group in the
Cottonwood district. This horizon is perhaps near the upper limit of Lytoceraa
joiurrtaJfl, and as judged from its associates it represents an upper part of the Valan*
ginian column in European chronology.
Lytoceraa fwicji (Trask)
(Plate 1J, fijwe 1; plat* 17, fl*ws 1)
Ammonites Batesi TRASS, Calif. Acad, Sci., Pr., vol, 1, 1855, p. 39; "Arbuckle Diggings,"
Shasta County—GABB, Paleont. Calif., vol. 1, 18S4, p. 87, pi. 13, figs.
IS, ICOj 10b ; locality as above.
Lytoceraa bateai, STANTON (in part), U. S, Geol, Surv., Bull. 133, 1806. p. 75, (not pi.
13, fig. 11; Wilcox ranch, Tehama County—ANDERSON, Calif. Acad. Sci., Pr.,
3d ser,, vol. 2,1902, p. 84.
This species is diseoidal, moderately robust, with almost circular eroeencction,
and distinctive ribbing, unlike that of any of its congeners. The surface ornamentation
ia clcarly given by Trask, aa follows (in part):
"Well defined costae on cach wborl, which appear uninterrupted, and on the last
convolution (whorl) are about one-tenth of an inch asunder; the ribs become more
approximate toward the ventral (dorsal) portion of the whorl; between the larger
ribs are seen smaller divergent rudimontiiry costae. . . ."
Although Gabb claimed to have figured "one of Trask's original specimens," his
diagnosis must be accepted with caution, since he confused at least three other quite
distinct forms with this aperies. Others have followed bis example.