No. 373. ] THE STUDY OF GRAPTOLITES:. 9 
have none. The explanation of this is found in the observa- 
tion that the initial part of the sicula of probably all Grapto- 
loidea is prolonged into a process (the virgula of Wiman, 
the hydrocaulus of Ruedemann, the nema of Lapworth), and 
that this process in the inward-growing rhabdosomes becomes 
incorporated as virgula, while in the outward-growing rhabdo- 
somes it remains outside (the nemacaulus of Lapworth). The 
virgula was originally hollow. This is indicated by Wiman’s 
observations in Monograptus and by the growth of gonangia 
from the central disk in Diplograptus, which gives the hydro- 
caulus the character of a stolon. 
The demonstration that the virgula or “solid axis” of the 
older authors is present only in a very restricted number of 
Graptoloidea is of great interest, as the presence of this organ 
has been considered as one of the principal characters of the 
graptolites and has even procured them the name ‘“ Rhabdo- 
phora.” The virgula, as identical with the hydrocaulus or 
nema of the sicula, is, in reality, present in probably all grap- 
tolites, for long filiform processes of the sicula have been 
observed in numerous genera (e.g., Mzeandrograptus, Didymo- 
graptus, Tetragraptus). Lapworth concludes from the presence 
of this process that all rhabdosomes were either fastened to a 
central disk, as in the Dichograptidze and Diplograptide, or 
directly by the nema to foreign bodies. 
We cannot leave the Graptoloidea without mentioning the 
important investigations of Tullberg (82), Jækel (90), Perner 
(94), and Giirich ('93,'96) on the shape and position of the 
thecze, and their apertures in Monograptus, which led to the 
division of the genus into the two subgenera Pomatograptus 
Jeekel (Monograpti reversi Girich) and Pristiograptus Jækel 
(Monograpti erecti), the interesting study of the phylogeny of 
the Dichograptidz by Nicholson and Marr (95), based on the 
shape of the thecæ, and the work of Miss G. L. Elles (97) on 
the relations of the subgenera Petalograptus and Cephalo- 
graptus. 
The knowledge of the structure and development of the 
rhabdosomes of the Retioloidea is still so uncertain that it can 
be passed here with the remark that a different number of 
