GRAPTOLITES OF NEW YORK, PART 2 
ture being reduced to a small pore at the apex of the spine. These spini- 
form thecae, though obviously adapted to defensive purposes, are not 
comparable to the spines of other graptolites here discussed, and appar- 
ently have no such bearing upon the phylogeny of the class as the spines. 
There are also thecae in early species of Dictyonema, Dendrograptus and 
other genera that by their acute form may have themselves assumed the 
function of defensive spines. 
Having traced the origin of the spines in the forms here under investi- 
gation, we will now turn to the bearing of the time of their appearance on 
the question of the stage of development of the spinose forms. Consider- 
ing the whole class of graptolites, the appearance not until Trenton time of 
the great majority of the spinose forms corroborates the observation made 
in regard to other classes, as the Trilobites, Brachiopods and Ammonites, 
viz, that the greatest development of spinose organisms occurs just after 
the culmination of a group, and, as this period clearly represents the begin- 
ning of the decline of the vitality of the group, the spines are to be taken 
as the visible evidence of this decadence. According to Beecher the 
appearance of spines indicates the paracme of a class. 
If we compare, however, the appearance of spines in the various phyla 
of the graptolites, it will be seen that it does there by no means begin 
everywhere either in the same astophyletic stage or correspond in its 
principal development to the paracme of the race. 
InTetragraptus pygmaeus, Didymograptus caduceus 
var. nanus and Phyllograptus anna mut. ultimus spinosity 
does not appear until just before extinction or in the last astophylogerontic 
substage. Perhaps this phenomenon can be accounted for by the fact that 
the races to which these forms belong, die out before the acme of the whole 
class is reached. They may then illustrate the principle established by 
Hyatt that gerontic characters appear first in the ontogenetic and phylo- 
genetic history of a group in the last stage and are progressively pushed 
into earlier stages. On the other hand we have the very early spinose, 
though thus far isolated Tetragraptus acanthonotus. 
