Hyatt.] 370 [June 7, 



In Steph. subcoronatum the contraction is also well marked as old 

 age advances, though here, as in Humphriesianum, the time at which 

 old age may begin varies greatly. In Blagdeni I was not able to 

 observe any very marked old age changes, except perhaps a tendency 

 to narrow the abdomen. 



In the Banksii variety of SlepJi. coronalum the old age changes are 

 well marked, the tubercles decreasing in size, and the ribs becoming 

 depressed and finally obsolete. In the planulum variety of coronatum 

 this retrograde tendency is carried out fully, appearing even in the 

 adult shell, so that the abdomen becomes very narrow, and the ribs 

 in some specimens have no tubercles, except in the earlier stages of 

 growth. The extreme changes in the individual figured by D'Or- 

 bigny, I have not observed, but his figures are doubtless correct, since 

 the indications of tlie obsolescence of the ribs, and the depression of 

 the angular sides in the normal variety, are very marked in much 

 smaller shells than those which he describes as having only undula- 

 tions on the side at the diameter of 230 mm., and that which he 

 figures as entirely smooth at the diameter of 486 mm. Kone of these 

 .are found in his collection, but probably exist elsewhere, although 

 he does not allude to them in this connection. Even at this enor- 

 mous size, 486 mm., the shell of the normal variety, i.e., that which 

 has the Blagdeni- or Pettos-like fcrm until a late stage of growth (see 

 D'Orbigny, Terr. Jurass., pi. 169, fig. 1-2, and pi. 168), retains the 

 lateral tubercles, though these are so close to the umbilical edge as to 

 give them an entirely distinct aspect. 



It will be observed that these old age metamorphisms of the indi- 

 vidual are not only correllative with those occurring in the planulum 

 varieties of Steph. coronatum, but they also resemble, in a measure, 

 the changes which take place in the evolution of the Steph. Bayleanum 

 out of the Steph. nodosum forms. This consists simply of a decrease 

 in size of the whorls. When it takes place, in an old specimen of 

 Steph. nodosum it is an old age degradation al change. When it takes 

 place at an earlier stage it produces forms intermediate between 

 nodosum and Bayleanum', and when at last it occurs at an early age, it 

 changes the quick increase in the growth of the whorls to a slower 

 rate, and produces the narrow whorls and discoidal form of Baylea- 

 num. It differs from the old age changes in not going to the extreme 

 extent of destroying the tubercles, ribs, etc. 



Quenstedt describes specimens, all of which must, I think, be re- 

 ferred to Brochii, in which the tubercles are lost at an early age, but 



