” 
IO THE AMERICAN NATURALIST. (VoL. XXXII. 
so-called virgulze and an “initial canal ” of the rhabdosome have 
been observed. 
The most interesting discoveries have apparently been made 
in regard to the third group of graptolites, the Dendroidea. 
These finely branching, plant-like graptolites, on account of 
the absence of a virgula and the occasional presence of a dis- 
tinct organ of fixation, which indicated a sessile mode of life, 
in opposition to the generally supposed floating habit of the 
Graptoloidea and Dendroidea, have been mostly separated from 
the other graptolites, and are often unhesitatingly united with 
the campanularians (cf. Zittel, 79). Their occurrence only as 
flattened films prevented, until recently, all attempts at closer 
investigations. The first indication of a more complicate 
structure was given by Holm (90) in his description of Déctyo- 
nema cervicorne. He found that the branches of the rhabdo- 
some in this species consist of vertical rows of thecze, which 
end in hayfork-like spines. Situated alternately on two sides 
of these spines are peculiar cup-shaped bodies (“ by-thecæ ”), 
which he compares to birds’ nests and supposes to have been 
gonangia. While Holm first isolated these delicate fossils from 
the limestone matrix, Wiman (95, '96) went further, applied 
the microtome to them, and obtained a complete series of thin 
sections of Dictyonema, Dendrograptus (?), and Ptilograptus. 
The important result of his microscopic analysis is the discovery 
that the morphological structure of the Dendroidea is much 
more complex than was suspected, and that there are not less 
than three different forms of thecæ, viz., thecæ proper, or 
nourishing individuals, alternating canals (Holm’s by-thecæ), 
which Wiman also considers as gonangia, and gemmating 
individuals. The following illustrations (Figs. 21—28), selected 
from a series of one hundred and twenty-five sections through 
a branch of Dictyonema rarum Wiman, show this diversity of 
the thecæ quite plainly. The nourishing individuals are de- 
noted by ¢, the gonangia by g, and the budding individuals 
by & Fig. 22 shows the opening of the gonangium on the 
right side ; Fig. 23, the ceasing of the theca ; Fig. 24, the 
growth of a new theca. In section 25 the budding individual 
' has produced three new individuals, the further growth of 
