LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

148 lower chetaceous deposits ijf california and oregon
In good specimens of this species secondary costae first appear on the young shells
near the ventral sons, and with growth these extend downward to the dorsum. In
mieh specimens, up to a diameter of 100 mm., the costae are simple linear ridges, often
acute, separated by broadly concave interspaces; with growth tbe costae become
broader and more rounded, and tho interspaces are relatively narrower. At a diameter
of 10(3 mm. the interspaces begin to become occupied by secondary costae, which
in more mature stages assume the rank of primary costae- These costal ridges remain
simple throughout, although some writers have believed that well-preserved
specimens show crenulated ribs.
Inspection of a large number of representative examples ia good preservation has
failed to confirm this view. In most if not in all eases in which crenulations appear,
critical measurements show departure from the type form.
The holotype of the species was tbe property of the California Academy of Scieneeft,
and was loat in the San Francisco fire of 1006. Many good specimens have
since been found, and are in the collections of the Academy. Many of them have
been carefully measured, and are found to depart but little from type, A representative
example, 110 mm. in diameter, gives the following results: width of umbilicus,
63.5 mm.; height of whorl, 33 mm,; width of whorl, 33 mm.; umbilical ratio,
0.486:1.
From the loss of the holotype it has become permissible to choose a lectotype for
tbe species. This lectotype (Calif. Acad. Sci. type Coll.) has the following dimensions:
greatest diameter, 166 mm.; width of umbilicus, 85.5 mm.; height of whorl,
$1 mm.; width of whorl, 51.5 mm.; umbilical ratio, 0.500:1.
The largest example of this species thus far seen was found on the North fork o(
Cottonwood Creek, near Ono. It had a diameter of about 14 inches, but Bhowed no
fluting on tbe ribs. Six or more good specimens of this species from the same district
are in the Museum of Paleontology, University of California, and many more arc in
the collections of the California Academy of Sciences, and in other collections.
The Stratigraphic range of tbe species is not definitely known, but insofar as it
is known the species is confined to the lower half of the Horsetown group and is
most abundant in the upper part of this half. The lectotype, with various other
examples of the species, was obtained at Locality 1347 (Calif. Acad. Sci.) in the Bald
Hills, eaat of Mitchell Creek, about 5 miles south of Ono, Shasta County.
A representative example, the property of the Academy of Natural Sciences of
Philadelphia, is figured on Plate 16 of this paper. It has the following dimensions:
greatest diameter, 135 mm.; width of umbilicus, 65 mm.; height of whorl, 40 mm.;
width of whorl, 40 mm,; umbilical ratio, 0.481:1. The locality from which this
example waa obtained, unfortunately, is not known, hut it seems to have come from
the Cottonwood district in Shasta County.
Lyioceras (Arganatilicerat), new subgenus
Genotype, Lyloceras argonauiarum Anderaon
Among the many subgeneric groups of Lyloccras (a. 1.) found in the standard
paleontological literature none seem adapted to include thiB type from the middle
Horsetown group of tho Shasta series, and similar forms described from other regions.
Probably this group should include Lyioeeta* bclliscptatum Anthula (189ft, p. 97,
plfl. 6 (5), 7 (0)) and perhaps &\sa Lytactrat ezoense Yabe (1903, p. 0, pi, 1, fig. 1) from
tbe Lower Ammonite beds of Hokkaido, Japan. In this group of lytoceratids tiie
Y&CTfci&MA mi \muyiifi.\ ciosa-aeeiiSon ol \Aw>t\ are frWungufiaVm&
features. The genotype of the group is also the holotype of the leading apecitB.
This is an incomplete, wholly septate whorl having the following dimensions: great-