LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

212 LOWER CRETACEOtffi DEPOSITS IN CALIFORNIA AND OREGON
species of Auttraliceraa bear tubercules, at least in their younger stages. Whitebouse
believed that Tropaevm waa achieved by the abrupt cessation of tiiberculation.
The number of species of this genus occurring in the Lower Cretaceous of California
is not known, but fragmentary examples of the genus indicate more than one- Examples
are not rare in the Argonaut zone and appear in strata just above it, but in
all cases they are found in cither Barremian or Aptian strata. The range appears
to be from upper Barremian to lower Gargoaian. Examples of the genua have been
found in the Lower Cretaceous of the Alaskan coast, near Cook Inlet.
Tropaewn percottalum (Gabb)
(Fbt* 71, flpj« 1; plat* 72. Astro t)
Criocerm percoalatus GABH, Paleont- Calif., vol. 1, 1864, P. 17, pi. 16, fig. 26; pi. 17,
fig. 28a; North fork of Cottonwood Creek, Shasta County (not "Aiuylocer&s
percoaUtlua" Gabb, vol. 2, 1869, p. 138, pi. 24, fig. 19, Voy collection).
Gabb's (1864, p. 77, pis. IS, 17) original figures and description of thia species
afford a fair conception of its form and other characters, although tbey are not at
all complete. His account (Gabb, 1889, p. 136a, pi. 24) included under thia name a
quite distinct species and genua. As no later figures and description of either form
have appeared, the earlier figures of "Crioceraa" percoalotua are selected as representing
Gabb's species. The second form included by him is described herein as
Shoeloccroi behemoth n. sp. Tbe present apecies Trepoetin percoatoium (Gabb) is
not uncommon in the Horsetown group in the Cottonwood district and has been
found as far south as Hayward, Alameda County. Good examples have been found
on Hulen Creek, Cottonwood Creek, Alderson Creek, Bee Creek, Mitchell Creek,
Roaring River, and McCarthy Creek, in all cases in nearly the same horizon. In its
general features it is not unlike T. b&twbanki (Sowerby). In form, coBtation, and
suture lines in its younger stages of development, it resembles Australiceraa, aa
suggested by Whitehouse, although it lacks the tuberculate riba belonging to the
latter. In its older stages it develops the thick, heavy ribs to which its name refers,
In mature shells the section of the whorl is aubquadrate, the ribs nearly straight or a
little flexed forward near the periphery in examples below a diameter of 5 inches,
and it tends to divide, either near tbe dorsal border, or in some eases above the middle
of the side; sides are flattened, Bloping toward the periphery. In older stages the
shell of the body-chamber departs a little from the earlier whorls. The largest
example seen has a diameter of 17 inches.
Shaaloeeraa Anderson, n. genua
The genotype of the present group 'IB its leading speeies 3ha*U)cerai californicum
nov. The specimen iB in the Museum of Paleontology, University of California,
and was mentioned by Gabb under tbe name "Ancyloceras percoatatua" as being "in
the cabinet of Mr. Voy, now deposited in the museum of the College of California."
In its final stages the shell of thia genus acquires the general form of Ancyfocerm,
leaving the early coiled stage and forming a straightened or gently curved shaft
which increases rapidly in section, and finally forming a thick crosier having a
narrowed recurved limb. The earlier whorls are sub-elliptical in section, and the
early part of the abaft retains thia form, but with growth it becomes less compressed;
in its body-chamber the shell becomes more inflated, and in the recurved limb it ia
much reduced in diameter and nearly circular in section; the shell here forma a
few strong costal ridges, more prominent on the ventral side. In its younger whorls
(at least in one species), there are costal ridges which arc interrupted on the ventral
border by the development of small bullae which soon disappear with growth.