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small 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, i. 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 Icevis 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 costae 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 retro versal 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. 143. 



