264 TRANSACTIONS OF THE [may 20, 
All the graptolites of this early horizon that may be included 
in the Rhabdopora, appear to be of the family Dichograptidz 
and are chiefly of the two important sections typified by Bryo- 
graptus and Clonograptus, the former with a distinct sicula and 
the latter devoid of this initial part; or with the sicula obscure, 
absorbed, or merged in the funacle. 
The succession of the Dichograptide in the Cambrian and 
Lower Ordovician is a good exemplification of increased con- 
densation of structure due to selection; for the many branched 
forms of the former are gradually replaced by the Tetragrapti 
and these by the Didymograpti of the Upper Arenig. As the 
branches of these graptolites diverge at a variety of angles in 
the different species, so a like variation in the attitude of the 
branches is found in the various species of Bryograptus in the 
Upper Cambrian rocks. Froma form belonging to this genus we 
may suppose that Dictyonema flabelliforme arose, for even when 
this species was flourishing and dominant, we do not find that 
the reticulating threads that knit the branches together were 
present on the primary and secondary branches, and even the 
tertiary branches are sometimes devoid of them. From this 
acquired habit of linking its branches together, or by the innate 
vigour of its constitution this species was enabled to overtop 
and dwarf its fellows, so that while the other Brytograptids are 
comparatively of puny size, individuals of this Dictyonema have 
been found eight inches in length,* and with nearly two hundred 
branches to the hydrosome. 
While the genus, from which we may suppose that Dictyo- 
nema sprang, had but a short life, Dictyonema was able to 
maintain itself in competition with the vigorous Dichograpti of 
of the Quebec Group and thus contribute a very respectable 
contingent to the Ordovician Fauna; and if the species referred 
to it from the Silurian (Upper) were truly of this genus it had 
4 wider range than any of the graptolitic genera. 
Section AT Navy ISLAND. 
The shore of this island where the fossils were obtained that 
form the subject of this communication has on its western side 
black and dark gray shales of the lower half (or more of Div- 
ision 3 of the St. John Group, exposed between high and low 
water mark. These ledges are swept clean by the force of the 
current of the river which passes here, and so furnish available 
opportunities for collecting the fossils which they contain, when 
the tide is low. The three lower zones of Division 3 are ex- 
*The Graptolites described by Heisinger, p. 21. 
