70 AMMONITIDZ. 
Body-chamber short, one-half to two-thirds of a whorl long. 
The sculpture on the convex portion is interrupted; in the geo- 
logically younger forms a more or less deep median furrow is 
sunken in, at which the ribs terminate in a tubercle. Aperture 
with a short lobate process on the convex portion. Lobes agree- 
ing with Tropites; much simpler in the geologically older forms. 
ARPADITES, Mojs., 1879. Periphery with a deep groove, some- 
times bordered by smooth or nodulous carinations ; ribs numer- 
ous, dichotomous from an umbilical nodosity. The older forms 
have entire saddles; the more recent ones are toothed to the 
summit. 7. Arpadis, Mojs. 
HERACLITES, Mojs., 1879. Body-chamber only occupying half 
a whorl; ribs strong, nodulous on the sides; periphery of the 
last whorl flattened, with two thread-like spiral lines; lobes 
distinguished by several irregular notches, but deeply truncate. 
T. Poschli, Hauer. 
SAGENITES, Mojs.,1879. Body-chamber occupying half or three- 
quarters of the whorl; ribs usually not interrupted at the per- 
iphery, crossed by very close spiral lines; saddles high and 
wide, branched, foliaceous; lobes branched; auxiliary lobes very 
small. 7. Giebeli, Hauer. 
GYMNOTOCERAS, Hyatt, 1877. 
Distr.—G. rotelliforme, Meek (xxxvi, 90,91). Trias; Nevada. 
The development of Ammonites Blakei, Gabb, and the char- 
acters of its periphery, separate it at once most decidedly from 
any species of Trachyceras. The development generally of a 
keel, or, in some varieties, of a raised periphery, over which the 
pilze do not pass, shows that this is a different genus, character- 
ized by a different mode of development. The septa are quite 
similar to those of Trachyceras, but it is very evident that in 
the Trachycerz the septa cannot be looked to for generic differ- 
ences. Great differences also occur in the amount of involution 
of the different species and in the development of their external 
characters. 
Clydonite. 
Body-chamber short; sutural line undulated; lobes and saddles 
simple, not dentate. 
CiyDonitTes, Hauer, 1860. 
Etym.—Kludon, the surge. 
Distr.—21 sp.; Upper Triassic; Europe, Himalayas. 2 sp.; 
Upper Cretaceous (described by d’Orb. as Ceratites. Difference 
from Ceratites is the lobes being simple, not crenulated). C. 
costatus, Hauer (xxxii, 27). C. delphinocephalus, Hauer (xxxii, 
28). 
Shell spiral, discoidal, whorls involute, ribbed ; sutures simply 
lobed, the lobes pointed. 
