WHITE.] TRIASSIC FOSSILS. ~ Le? 
‘‘The specimens are too fragmentary to tell with any certainty the 
species ; and even the genus ought perhaps to be considered doubtful, 
because the whole outline of the suture was not seen. ‘The runzel- 
schicht,’ however, was so marked that this appeared to show them to be 
identical with Arcestes. ‘The septal sutures are, however, not entirely 
unlike those of Gymnotoceras.” 
There are only two specimens of this species in the collection, both of 
which, it is true, are too imperfect to afford means for the satisfactory 
characterization of the species. The characters which are shown, how- 
ever, are quite sufficient for its identification and its discrimination from 
other forms. For the convenience of future reference and the character- 
ization of the strata from which it comes, I have therefore applied the 
above specific name. 
This shell, like those of the three species of Meekoceras herein des- 
cribed, is compressed-discoidal; the siphonal side rounded in the young 
and apparently a little angular in the adult, but the latter feature is not 
plainly shown in either of the specimens; umbilicus open, but compar- 
atively narrow; sides of the volutions flattened, those of the outer one 
of fully adult shells almost flat or only slightly convex, except near the 
outer and inner sides; inner border abruptly shouldered to the next 
inner volution ; involution very great in the young, but it so lessens 
with the growth of the shell that the outer portion of the living-cham- 
ber of adults does not embrace by more than one-fifth its own width 
that of the next volution within. The examples do not show the char- 
acter of the sutures satisfactorily, but the lobes and cells are both seen 
to be constricted about the middle, the ends of the former oblique, but 
digitate, as in Meekoceras, and the latter plain as in both that genus 
and Ceratites. Surface conspicuously marked by numerous longitudi- 
nal or revolving abruptly raised lines, which enlarge into ridges in the 
adult and which are separated by spaces a little wider than themselves 
at medium growth of the shell, but becoming wider in the fully adult. 
This species is much larger than either of the other Cephalopods de- 
seribed in this article; the extreme diameter of the coil at fully adult 
size being not less than 180 to 200 millimeters, and the vertical diameter 
of the outer portion of the living chamber about 80 or 85 millimeters. 
Position and locality.—Triassic strata, member D of the section at 
locality No. 1, shown on a previous page, Southeastern Idaho; collected 
by Dr. A. C. Peale. 
ARCESTES oe 
Fragments of young examples of two more species of Cephalopods 
were obtained by Dr. Peale from member D of the section at locality 
No. 1, which are too imperfect for specific determination, but which seem 
to be referable to Arcestes. One of these forms is so very like young 
examples of A. gabbi Meek, vol. iv, U.S. Geol. Sur. 40th Parallel, p. 
121, pl. X, figs. 6, 6a, and 6 b, as to leave upon the mind a strong im- 
pression of their identity. Such identity, however, would seem to neces- 
Sitate a reference of the strata containing these forms to the same hor- 
izon as that of the Trias of Nevada and California, of which we have no 
other fact in support of, and many against such a reference. 
The species embraced in the foregoing descriptions are far the most 
important of those that have been yet found in the horizon which is in 
this article referred to the Middle Trias. Indeed the only other known 
forms or indications of other species are some fragments among the 
