581 
sinall fossil the gerontic umbilical perforation is much larger and 
wider in proportion. 
One in fact begins to find the same difficulties in the application 
of the purely mechanical theory of the origin of the gerontic dorsal 
furrow here that was mentioned in accounting for the origin of the 
dorsal furrow in the nepionic stages of the close-coiled Nautiloids. 
My opportunities and materials do not permit me to discuss the 
subject intelligently but merely to note the facts. 
One fragment of a volution or an arm, apparently of this species 
and identical in every way with the other two of the same lot in 
ornamentation and form, has, however, a gibbous dorsum. 
It is either not a Ptychoceras or it is the paragerontic substage of 
this species after it has passed the age in which the gerontic con- 
tact furrow is present, or else, as I have suspected from the exami- 
nation of other species, any species of Ptychoceras may have modi- 
fications that would place it in the genus Hamulina, 7. ¢e., some 
specimens may not be closely appressed in the gerontic stage and 
may not have the gerontic contact furrow. 
Diptychoceras.* 
The single species described by Gabb as Diptychoceras levis is of 
interest in this connection as a further modification of Ptychoceras. 
It has in its ephebic stage a straight arm occupying the same 
position with relation to the younger or first straight arm as that of 
the gerontic arm of Ptychoceras. ‘That this is the ephebic stage is 
shown not only by the presence beyond it of the third straight arm, 
but also by the presence on the second arm of costz that incline 
orally in passing on to the venter. 
The gerontic characteristics of Ptychoceras are therefore only in 
part, not as a whole, carried back into the ephebic stage of Dipty- 
choceras. The gerontic stage or third arm in the ontogeny of the 
shells of this species is similar to that of Ptychoceras and this has 
its own gerontic characters. The tendency to the peculiar mode 
of growth first found in the gerontic stage of Ptychoceras, the 
closely appressed retroversal straight limb is, however, inherited in 
the ephebic stage of Diptychoceras. 
It would be interesting to follow out the history of the impressed 
zone in the gerontic stages of shells of this species, but I have no 
*Gabb, Pal. Cal., ii, p. 148. 
