winte. "TRIASSIC FOSSILS. me Lal 
the middle a little obliquely rounded, inner cellincomplete; the outer lobes 
very small, usually notched at the end, pointing directly backward in the 
inner volutions, but in the outer septa they diverge a little from each 
other; middle lateral lobes of about the same width as the outer cell, 
bearing at its end about five small digitations; inner lobe smaller than 
the middle one, but similar to it and bearing about four small digita- 
tions; auxiliary lobes and cells apparently entirely wanting. Surface 
of the outer volution nearly plain, or bearing faint indications of trans- 
verse nodes or ribs, which upon the inner volutions are numerous and 
more distinct. 
Diameter of the coil of the largest example in the collection, 60 mil- 
limeters; extreme vertical diameter of the cuter volution, 22 millime- 
ters; transverse diameter of the same, 12 miilimeters. 
Position and locality.—Triassic strata; locality No. 2 of the preceding 
pages; Southeastern Idaho, where they were collected by Dr. A. C. 
Peale. 
Professor Hyatt makes the following remarks upon I. aplanatum: 
“This species belongs to that group of this genus in which the shells 
have but a slight amount of involution, and possess also the narrow 
and numerous whorls which are the invariable accompaniments of this 
characteristic in all the Ammonitoids and Nautiloids. The resemblance 
to Amm. parcus von Buch* is very close so far as the outlines of the 
sutures are concerned, but the sides are too flat, the umbilical shoulder 
very abrupt, and the siphonal side flattened. There is an equally close 
resemblance to Ceratites carbonarius Waagent, but here again the 
siphonal side is flatter and the involution greater, the last whorl at the 
Same size as in the largest one figured, being broader on the side. The 
septal sutures are, however, very similar. 
“The young, when of the same size, are very similar to the figure of 
Goniatites levidorsatus Gabb,t but when larger, as in Meek’s figure,§ 
very considerable differences appear. The levidorsatus being a heavily 
ribbed shell, even when quite large, with a rounded abdominal side; 
though here again I doubt if the sutures differ much. 
“The young shell of species ‘A’ [JZ. aplanatum] has rounded smooth 
whorls, which increase very slowly by growth, and are full half an inch 
in diameter before the sides and abdomen (siphonal side) begin to show 
the flatness which characterizes the adult. At about this size also the 
whorls grow perceptibly broader in proportion on the sides, and the ~ 
amount of involution increases gradually until it covers about one-third 
of the side, in a specimen about two and a half inches in diameter; the 
umbilical shoulder also changes at the same time, becoming abrupt and 
subangular. 
“In some specimens there are indications of nodes on the sides, and 
in some the young until a late period are distinctly ribbed, the ribs being 
thick straight folds, reaching across the sides, but not up on to the 
siphonal side (abdomen). 
“The age at which the serrations of the lobes.appear could not be 
seen, but great caution should be exercised in this group in deseribing 
these characteristics, since they are liable to disappear with the removal 
of the shell on account of their shallowness. Thus, a full-grown shell 
may be readily mistaken for a Goniatite ; or the young, before the serra- 
tions appear, for one of that genus.”—(A. H.) 
* Ueber Ceratiten, Avhand. Akad. d. Wissensch, 1848, pl. 4. 
t Memoirs Geol. Sur. India, Pal., vol. iv, art. 4, pl. 1. 
{ Pal. Geol. Surv., California, vol. i, pl. 3. 
§Geol. Expl. 40th Parallel, vol. iv, pl. 10. 
8H 
