66 AMMONITID&. 
ARCESTES, Suess, 1865. 
Distr.—130 sp., Trias; several sp. Permian, and Carboniferous 
of India; 1 sp. Trias, N. Caledonia. Arcestes tornatus, Bronn 
(xxxviil, 20, 21). 
Shell, as a rule, smooth, sculptureless, seldom with longitudinal 
strie (Tornati); body-chamber long, taking up one to one and 
one-half whorls. Whorls strongly involute. Aperture usually con- 
tracted by the border being reflected inwards or by internal ridges. 
Lobes strongly incised (laciniated), so that the saddles merely 
consist of a slender stem with numerous approximated hori- 
zontal branches, which in turn are divided into smaller branchlets. 
Many forms have internal nuclei with an open umbilicus, and 
a terminal whorl with a callous closed umbilicus. 
ARCESTES (restricted). Last whorl of the spire more or less 
modified in form; umbilicus closed by a callous deposit. 
SPHINGITES, Mojs., 1879. Surface of the last whorl sillonated ; 
umbilicus open. 
CLADISCITES, Mojs., 1879. Section of the whorls subquadran- 
gular, their form not modified in the adults. The character of 
the suture recalls the Pinacocerw; second lateral lobe deep, 
saddles slender. much divided. 
JOANNITES, Mojs., 1879. Sutural lines like those of Cladiscites, 
but the other characters those of Arcestes. The sutural line is 
arcuated, lobes and saddles partly divided in pairs, much toothed. 
DIDYMITES, Mojs., 1873. External form and length of body- 
chamber same as in Arcestes; shell with sharp lines of growth 
and plicate wrinkles throughout the whole length of the body- 
chamber to the aperture; on the inner convex surface of the 
shell there is a median furrow; the last whorl is constricted near 
the aperture. 
The sutural lines of the septa are formed of few-toothed 
saddle pairs, which often alternate with single saddles. These 
saddle pairs, as is shown by projection of the spiral, correspond 
each to two saddles in the other genera of Ammonites. 
Didymites contains but a few Triassic forms. Didymites 
angustilobatus, Hauer (xxxix, 32, 33). 
CycLoLosus, Waagen, 1879. 
Distr.—C.Oldhami, Waagen (xxxvi, 93,94). Paleozoic ; indin: 
Shell spiral, smooth, with a small deep umbilicus ; ‘whorls 
numerous, somewhat compressed, with a broadly rounded exter- 
nal side, deeply embracing each other so as to envelop the 
preceding whorl, entirely covered on the sides with not very 
numerous contractions of the shell (varices), indicating very 
likely the remnants of old apertural margins. The form of these 
varices is somewhat falciform, with a strong bend towards the 
front in the middle of the sides of the whorl, and being bent 
