AMERICAN PALEOZOIC CEPHALOPODS 
231 
One of the segments of the siphuncle, at the base of the specimen, 
exposes the deposit lining the interior of the siphuncle, and the 
abrupt constriction of the siphuncle where it passes through the 
septum. Specimen No. 22920, in Walker Museum at Chicago 
University. A fragment of a siphuncle, showing the abrupt con- 
striction of the siphuncle, is referred to the same species. It 
(Plate XXXVIII, Fig. 3) is numbered 22916 in the same museum. 
Both are from the Niagaran. 
Lemon t specimen. — A specimen (Plate XXXVII, Fig. 3) ex- 
posing the structure of the interior of the conch, from the Niag- 
aran at Lemont, Illinois, is numbered 22917 in Walker Museum. 
29. TRIPTEROCERAS HYATT 
Genotype: Orthoceras hastatum Billings. Geol. Surv. Canada, Rep. 
Progress for 1853-56, published in 1857, p. 333 
Original Description: Tripteroceras has similar forms and 
sutures to Eudoceras, but the lateral saddles are acute. The ven- 
ter is flattened, and broader than the dorsum, which forms the 
apex of the subtriangular section. The siphon is ventral and 
nummuloidal, and the whorl arcuate in the young, though straight 
in the full grown, and the aspect altogether distinct from the 
shells of Eudoceras. The young are similar to the adults of 
Eiidoceras.^"^ 
The genotype is Orthoceras hastatum Billings. An examina- 
tion of the types in the Victoria Memorial Museum at Ottawa, 
Canada, shows that these types include two species, one of 
which is described here as Tripteroceras pauquettense. 
The siphuncle of Orthoceras hastatum was described by Bil- 
lings as small, close to the centre of the ventral margin. The ab- 
sence of any further description of the siphuncle suggests that, 
at the time the original description of this species was being pre- 
pared, the specimen here called Tripteroceras pauquettense did 
not form part of the type series of Orthoceras hastatum, but was 
added subsequently. The siphuncle of typical Orthoceras has- 
tatum appears to be narrow, and not nummuloidal. Omitting 
this one word, nummuloidal, the remainder of Hyatt^s descrip- 
tion of the genus Tripteroceras will stand, with typical Ortho- 
ceras hastatum as the genotype. 
The conch is strongly depressed dorso-ventrally. The cross- 
section is depressed subtriangular, with the ventral side distinctly 
Hyatt, A., op. cit., p. 287. 
