292 
IOWA ACADEMY OF SCIENCE 
SOME UNIQUE NIAOARAN CEPHALOPODS. 
A. O. THOMAS. 
Among the paleontological collections at the State University 
of Iowa are some unique cephalopod remains. It is the purpose 
of this paper to call attention to a few of them and it is "hoped 
that at some later time the rich Niagaran fauna of which these 
are a part shall receive more extensive treatment. 
The specimens at hand consist chiefly of the siphuncles of 
several orthoceracone species. They are of the nummuloidal type 
and are usually strongly silicified. They occur at nearly all 
horizous in the Niagaran but most frequently in the Hopkinton 
or Delaware stage. Specimens are never abundant. 
The beautiful and rare pearly nautilus of our moderu seas is 
the nearest living relative of these extinct species. From our 
study of zoology it will be recalled that the coiled shell of the 
nautilus is divided by transverse partitions (the septa) into 
chambers. The septa are concave, their concave surfaces facing 
the opening or aperture of the shell. The animal lives in the 
body chamber,, a cuplike cavity above the last septum. An ex- 
tension of the bo(fy, known as the siphon, passes backward from 
the body chamber through perforations in the septa. In the 
modern nautilus the perforation in the last septum is but a 
millimeter or two in diameter and becomes gradually smaller as 
it is traced back through earlier and earlier septa. On the con- 
vex or posterior surface of each septum the shell substance is 
built out into a short tube around the siphonal perforation. 
This series of disconnected tubes is called the siphuncle. In the 
modern nautilus each segment of it extends only about one- 
third the distance across the air chamber into which it projects. 
The Niagaran cephalopods which are here considered were 
straight-shelled tapering forms much as our coiled pearly nau- 
tilus would make were it possible to uncoil it into the form of 
a straight cone. Curved, loosely coiled, and even closely coiled 
species were contemporaries of the straight-shelled ones but a 
few of the straight species only will be discussed in this paper. 
The slender discontinuous siphuncle of the pearly nautilus 
is but a vestige of the large continuous siphuncle of its extinct 
Niagaran ancestors. Moreover, it is quite likely that the siphon 
