OF CENTRAL CANADA PART IV. 227 



free-floating marine types, living in colonies of individuals which 

 secreted in common a horny or chitonous cellular support. The 

 latter, in a more or less fragmentary condition, has alone been 

 preserved, forming markings or impressions, mostly in argillaceous 

 slates. It is technically known as the " stipe." Most commonly it 

 presents a narrow, linear shape, " toothed " or serrated along one or 

 both of its edges. Frequently, these linear stipes bifurcate, and in 

 some forms (Rastrites, &c.) become partially enrolled or even spiral, 

 and assume in others a leaf-like form. Occasionally, the lateral 

 serratures are obliterated by transverse compression. These serra- 

 tures are the mouths or openings of minute cells, and thus much 

 resemble those of modern sertularians. They are pointed or even 

 mucronate in some genera, and obtuse in others. A somewhat 

 prominent thread-like line runs up the centre of the stipe (or along 

 the outer edge in the forms with one row of serratures) and often 

 projects beyond the stipe. This thread-like line is known as the 

 axis. Where its projecting extremity, or in bifurcating forms the 

 united extremities of two axes, forms a sharp or blunt point, this is 

 known as the "radicle" or "sicula." In some examples from the 

 Quebec slates of Point Levis, several bifurcating stipes radiate from 

 a common centre around which there appears to be a thin connecting 

 membrane or supposed " float." In the leaf-like forms from this 

 locality, as first pointed out by Professor Hall, of Albany, two, or 

 more properly four, stipes were united originally in a cruciform mode 

 of structure, although now generally separated. 



Graptolites may be arranged under five groups. These comprise : 

 (1) Monoprionidians* with single stipe celled on one margin only 

 (e.g., Monograptus, Spirograptus, Rastrites) ; (2) Dichoprionidians* 

 with dichotomously-branched stipe, celled on one margin, only (e.g. T 

 Didymograptus, Tetragraptus, Loganograptus, &c., all Lower 

 Silurian) ; (3) Metaprionidians^ stipe bifurcating, with single row 

 of cells on the separated portions of the forks, and a row on each 

 margin where the forks come together (e.g., Dicranograptus, Lower 

 Silurian) ; (4) Diprionidians, with cells on each side of stipe (e.g., 

 Diplograptus, Climacograptus, Phyllograptus) ; and (5) Retioprioni- 

 dians, with comparatively broad, bi-serrated, stipe, net-veined or 

 dotted on the surface (e.g., Retiolites, Retiograptus). 



* TrpiovtoTos, serrated, saw-like. 



t Mera, between, intermediate as regards the group. 



