932 THE AMERICAN NATURALIST. (Vor. XXXVI. 
under that generic name. When the spine has been fully 
developed, the fold in the mantle which caused it may gradu- 
ally diminish in size, with the active resumption of the shell- 
building process, until it finally disappears. In this case the 
spine is gradually closed on the apertural side, thus becoming 
symmetrically developed (Fulgur, Hemifusus). In some cases, 
however, the fold of the mantle appears to be lost abruptly 
when the shell-building process recommences. In such cases 
the apertural side of the spine remains open, becoming entirely 
external by the flooring over of the emargination produced by 
the spine in the shell margin. This type is most characteristic 
of the spines forming the varices of Murex. 
Through a process in acceleration in development, which will 
be more fully discussed below, the spines may be crowded back- 
wards, z.e., appear earlier and earlier, thus condensing the pre- 
ceding stages more and more until finally some of them are ` 
dropped out altogether. Thus it will eventually happen that as 
in Fulgur carica of the recent fauna the keeled and smooth stages 
are dropped and the spines follow immediately upon the tuber- 
cles, and to some extent even encroach on these. A gradual 
passage from the true tubercles to the true spines is thus pro- 
duced, and it becomes practically impossible to determine where 
one type ends and the other begins. This feature may be readily 
observed in the various species of Hemifusus, in most of which 
varieties occur showing all stages from the widely separated 
tuberculated and spinous stages to those in which spines and 
tubercles grade into each other without allowing a line of 
demarcation to be drawn. In trochoid and other shells in 
which the whorls embrace up to the angulation, the spines, 
if present, are either imbedded in the succeeding whorl and 
more or less covered up, or else they are progressively 
resorbed as the new whorl increases. In Melo and some 
other genera the spines project upwards and are generally 
unclosed on the apertural side. This produces the spiral 
“corona,” so striking a feature in some shells. Finally, the 
remarkable apical character of Vetus (Cymbium) proboscidialts 
uld be mentioned. In this a depressed, smooth, central, 
apical area occurs, due to a secondary deposit which covered 
