
938 THE AMERICAN NATURALIST.  [Vow. XXXVI. 
producing a consequent obscurity of the canal. The last stage 
in gerontism is observed in the loss of the power to coil, which 
is at first expressed in the separation of the inner lip from the 
columella ; and if any further coiling occurs, in the loose appo- 
sition of the whorls and the consequent formation of an 
umbilicus. These features. never go very far in the normal 
gerontic individual, for death intervenes and ends the process. 
Among the highly specialized types of gastropods uncoiled 
phylogerontic species are relatively uncommon. Perhaps the 
most striking example in a highly specialized group in which 
such a want of regular coiling does exist in the adult is Ver- 
metus. This, as is well’ known, has all the characters of a 
Turritella in its young stages, showing its derivation to be 
from that group. In the adult, however, it loses the power to 
form a regular spiral, turning and twisting in all directions and 
completely uncoiling at times, so as to make these portions of 
the shell almost indistinguishable from worm tubes such as 
Serpula. In primitive gastropods non-coiling, through the loss 
of the power to coil, appears to be the normal expression 
of old age, or gerontism, since in most cases the coil is the 
only feature which can be modified. 
Shells with the final portion of the coil unrolled are well 
represented in the Ordovician. They even occur in the Cam- 
brian, as indicated by Platyceras primevum Billings from 
the Lower Cambrian. From what is known of this little shell 
it appears that it represents a strongly umbilicated, low-spired 
naticoid type, with a portion of the last whorl free. If, as 
appears to be the case, the early whorls are normally coiled, 
and only the later coil loosely, this little shell represents 
the first phylogerontic type appearing side by side with the 
naticoid radicle of the entire group of gastropodous mollusks. 
Leaving out the loose coiling Eccyliomphalus and Eccyliop- 
terus as of questionable affinities, the first well-pronounced 
example of a phylogerontic type with the final whorls not coiled 
is found in the Ordovician pleurotomarioid genus Lophospira. 
This genus, which comprises mainly normally coiled umbili- 
cated forms, contains a few species which have become phylo- 
gerontic and show various degrees of non-coiling (Fig. 14). 
