No.373-] | THE STUDY OF GRAPTOLITES. 5 
demonstrates that a septum is formed, where the theca, 
instead of sprouting on the opposite side of the adjacent older 
thecze (¢1 ¢2), spring from the same side as the latter did (¢4 Z5). 
Holm (95) obtained most valuable information on the internal 
structure and development of some other important genera of 
the Graptoloidea; vzz., of Didymograptus, Tetragraptus, and 
Phyllograptus. The most interesting result of his researches 
is the demonstration of the conformity in the first stages of 
development of the rhabdosomes in these genera and the Diplo- 
graptide. It is probable, therefore, that the earlier develop- 
Fie. 5. E eae aint anti- BI side (Wiman). 
Fic. 6 s Toérnquist : anti-sicula side (Holm). 
Fic. 7. — Tet: a pares Hall: from the proximal end, with the sicula turned 
upwards, showing the aperture of the sicula, left and right thecæ, and the thecz of 
the 
Fic. 8. — Phyllograpius angustifolius Hal: — me perm. mo with the sicula side 
turned upwards, showing in th Pres. ed pst of the 
sicula, on each side of this th f the lef 
Y 
ment of all Graptoloidea was the same, and that it consisted in 
the formation of only one bud on one side of the sicula. 
Fig. 5 (from Wiman, '95) shows the initial part of a Dichograp- 
tid, and Fig. 6 (from Holm) of Didymograptus minutus. Both 
figures serve to illustrate the diverging growth of the first two 
thecæ, which produces the characteristic bifurcation of these 
forms. The repetition of this process (Fig. 7) in Tetragraptus 
leads to the formation of four branches. The same takes place 
in the development of Phyllograptus (Fig. 8), “only that the four 
branches are disposed near each other and form a single, cru- 
ciform, four-winged, longitudinal septum.” 
The conformity of the rhabdosomes of all Gtaptoloides has 
been made probable by these investigations (the Leptograp- 
