4 THE AMERICAN NATURALIST. [Vov. XXXII. 
in Monograptus lies alongside the sicula and grows at once in an 
opposite direction (Fig. 1, z). From this theca grows another. 
The continuation of this process and the arranging of the thecze 
in one series along the virgula produces the monoprionidian 
thabdosome of Monograptus. Also from the sicula of Diplo- 
graptus, as Tornquist (93) and Wiman found almost simultane- 
ously, there sprouts but one bud, and Diplograptus is, therefore, 
also monoprionidian. The first theca, however, grows at first 
towards the aperture of the sicula and then bends in the same 
Mi 
Y 
——— 
4 
In Figs. 1-8, s = initial part of sicula ; $1 = apertural part of sicula ; v= virgula; Z= theca. 
Fic. 1. — Monograptus dubius Suess : sicula end from sicula side (Wiman). 
Fic. 2. — Diplograptus sp.: sicula end from sicula side (Wiman). 
Fic. 3. — The same : a later stage from anti-sicula side (Wiman). 
Fic. 4. — Climacograptus kuckersianus Holm : sicula side (Wiman), 
direction as the thecze in Monograptus (Figs. 2, 3). The theca 
are arranged in two series, thus producing the diprionidian 
aspect. The supposition that the diprionidian graptolites con- 
sist of two coalescing monoprionidian branches, the double 
virgula and the double longitudinal septum of the older descrip- 
tions, has thus been refuted; the observations of Tullberg, 
Tornquist, and Wiman prove that not even a single longitudinal 
septum is always present, and that, if one is present, it does not 
necessarily extend throughout the entire length of the rhabdo- 
some. Fig. 4, representing a rhabdosome of Climacograptus, 
