8 THE AMERICAN NATURALIST. [VoL. XXXII. 
6. After this first generation of rhabdosomes has reached a 
certain age a second generation of gonangia begins to grow 
(Fig. 19). “ 
7. The siculæ formed in these develop into a new verticil 
of rhabdosomes. 
The result of this mode of development is a colony which 
consists of different generations of rhabdosomes, recognizable 
by the different lengths of the latter (Fig. 20). An espe- 
cially remarkable feature of the colonies is the position of the 
sicula at the distal end of the rhabdosome in regard to the 
central disk. The explanation of this peculiar position of 
the sicula is found in the observation that the first theca turns 
away from the aperture of the sicula and grows towards the 
apical part of the latter, or towards the central disk, thus forc- 
ing the whole rhabdosome to grow backward, so to say, towards 
the center of the colony. Wiman’s figure of the initial part 
of the rhabdosome of Climacograptus and Gurley’s figure of a 
colony indicate a mode of growth similar to the one described, 
only that in Climacograptus the colony apparently consisted of 
only one (the primary?) rhabdosome, as perhaps all siculæ became 
detached and developed colonies of their own. Also in Mono- 
graptus the same mode of growth prevailed. In Phyllograptus 
the rhabdosome grows also in an opposite direction to the sicula, 
but no virgula has been found by Holm, and the mode of fixa- 
tion of the rhabdosome is still doubtful. 
In the other genera of the Graptoloidea, however, it has been 
found that the first theca grows more or less in the direction 
of the sicula. As a result, the sicula remains near the central 
disk, at the proximal end of the rhabdosome, and the latter 
grows distally. In the Dichograptide the whole colony is 
formed by dichotomous branching from one sicula, which 
remains at the center of the colony. 
With the two different directions of growth of the rhabdo- 
somes from the siculz the presence of a virgula as rod in the 
thabdosome is closely connected ; for, by the observations of 
Tullberg, Tornquist, and Holm, the fact has been established 
that only the Graptoloidea with inward-growing rhabdosomes 
(Diplograptide, Monograptus) have virgulz, while the others 
