190 PSEUDOCERATITES OF THE CRETACEOUS. 
my experience goes they are quite distinct. Retardation occurs in 
cephalopods in phylogerontic forms. Thus in this phylogerontic genus 
the young is a highly modified, compressed involute shell in the neanic 
stage; the adult in some species like guadalupe and the Kuropean depressum 
may have additional modifications ending with a paragerontic stage also 
tuberculated, but with peculiar broad venter and lateral nodes. In other 
species, obviously closely connected, like pseudosyrtale, newberryi, planum, 
and European congeners like grossowwret and milleri, the first appearance of 
tubercles occurs later in the ontogeny and they are present in their full 
development only in the latest ephebic substage or gerontic stage. The 
same law holds also in the series leading from syrtale through intercalare 
and stantoni to whitfieldi. In some forms of syrtale and intercalare the 
three lines of tubercles appear eartier than in the typical forms of the same 
species and in placenta. 
In stantont and pseudoplacenta this later appearance of tubercles 
becomes invariable and correlates with the disappearance of the centran 
line and a tendency to decrease in size of these ornaments on the venter 
and umbilical shoulders. 
In whitfieldi this tendency results in the total disappearance of the 
tubercles in a large number of shells, three lines of tubercles being present 
only in a very few shells and always of very small size, plainly individual 
reversions. 
The sutures are simpler in outline and continue persistently to hold the 
syrtaloid outlines, and are easily separable, except in extreme age, in all of 
the guadalupan series, including not only the stouter forms like guadalupe, 
but the highly compressed shells like planwm. In the series leading up 
to whitfieldi there is a marked gain both in complexity of outline and in 
gerontic characters. This is apparent in intercalare and placenta, which still 
retain the three lines of tubercles, and is still more pronounced in stantoni 
and pseudoplacenta and culminates in whitfieldi. In these last the sutures are 
similar to syrtale only in the neanic stage, and in whitfieldi they become 
more rapidly complex in ontogeny than in other species and overlap to 
such an extent that it becomes more difficult to separate them throughout 
the ephebic and gerontic stages. This overlapping is a purely gerontic 
character, and the species that show it during the ephebic stage are 
therefore phylogerontic and to this extent degenerative in spite of the 
