
930 THE AMERICAN NATURALIST. [Vor. XXXVI. 
interfere with the comfort of the animal. Accompanying the 
formation of this notch is generally a change in the outline 
and ornamentation of the last whorls, as will be discussed 
further beyond. This type of structure is well shown in Melon- 
gena melongena (Fig. 9) and M. patula of the modern trop- 
ical seas, and may well be termed a * melongenoid" type of 
growth. A striking modification of this type is seen in Strom- 
bus, where it appears only in the adult and is accompanied by 
the formation of a lip, and by fingers in pteroceroids.! This 
may be termed a ** stromboid " type of growth. It appears peri- 
odically in Cassis, where it has the value of varices. An extreme 
type, the “cyprzoid,” is seen in Cyprza and Ovulum, where 
the spire is entirely covered in the adult. Still another type, 
characteristic of the Eocene Clavilithes and related genera, is 
produced by the flattening of the top of the posterior canal and 
the production of a flat sutural shelf, which however is unaccom- 
panied by an increase in the amount of embracing of the whorls. 
This shelf thus runs like a regular spiral path around the spire 
part way, in rare cases almost all the way, up to the apex. 
This type deserves to be specified as the *clavilithoid " type 
of growth. In extreme cases it spreads out laterally into a 
flange, which in certain species is broken up into blunt, 
vertically flattened spines (Fig. 18). 
One of the most pronounced modifications of the aperture 
is the formation of an anterior notch at the point most distant 
from the apex. This notch which lodges, and is due to the 
formation of an anterior mantle fold which serves the purpose 
of a 'siphon, i is in many specialized types drawn out into a long 
slender canal, which finds its most perfect development in 
Fusus and in certain Murexes, where it is occasionally trans- 
formed into an almost closed tube, from the apposition of the 
upper borders. This type of modification makes its appear- 
ance in the Ordovician genera Subulites and Fusispira, where, 
however, the canal is more suggested than actually developed. 
Ornamentation of the Whorls. — The simplest types of what 
for want of a better term we may call ornamentation of the 
1 From unpublished studies on Strombidz, by Miss I. H. Ogilvie, it appears 
that the Pterocera has a polyphyletic origin. 
