
928 THE AMERICAN NATURALIST. . [Vor. XXXVI. 
the representative in the Etcheminian limestones of the Atlan- 
tic coast region. That this species is derived from the round- 
whorled Straparollina remota of the same horizon seems probable 
on comparison of the species, and this is in line with the theo- 
retic consideration which derives the angular-whorled forms 
from more primitive round-whorled species. 
When the angulation appears far down on the whorl, so that 
the suture of the next whorl touches it, a trochoid shell is pro- 
duced, which varies in outline from the gently tapering form of 
many Cerithiums, etc., where the basal angle is very obtuse, 
through the flat-based Trochus, where the angle is strongly 
acute, down to Xenophora with concave base and overhanging 
lateral margins, where the acuteness of the angle is extreme. 
When the angle is relatively far up on the whorl, so that the 
body portion is the largest, we have again two types of spires 
produced. When the succeeding whorl joins the earlier one 
below the angle, a turreted or terraciform spire is produced, in 
which the length and slenderness of the spire depends on the 
amount of the embracing of the whorls and the obtuseness of 
the shoulder angle. The long graceful spire of Fusus toreuma 
and its near allies is a result of a pronounced slope of the 
shoulder, together with a very moderate degree of embracing 
of the whorls, or what might be called a slender coiling. The 
strong contraction of the body of the whorl below the angle 
accentuates the slenderness of the spire, producing what is 
generally called a depressed suture. In some other species of 
Fusus all these features are less accentuated, and the spire as 
a result becomes more condensed and relatively stouter, thus 
losing some of its gracefulness. In other gastropod shells we 
can study all degrees of condensation of the spire owing to the 
increased embracing of the whorls, the flattening of the shoulder 
even to right angles with the axis of the spire, and the assump- 
tion of a cylindrical form by the body of the whorl. When 
the later whorls embrace the preceding ones up to the shoulder 
angle, a uniformly sloping trochoid spire is produced, the degree 
of slope of which depends on the angle which the shoulder 
makes with the longitudinal axis of the spire. Thus we may 
have every gradation from the long slender spire of certain 
