606 A POPULAR EXPUSITION OF THE 
near the Virgularize or sea-pens, belonging to the lower of the two 
great orders or divisions in which modern forms of this class are 
mostly arranged. It should be observed, nevertheless, that some 
naturalists divide the Potypirera into three Orders—Hydroida, 
Aleyonaria and Zoantharia (or groups with other names synonymous 
with these)—and place the graptolites (with the modern Sertularia, 
&c.,) in the first order. Agassiz, again, removes this order to the class 
ACALEPHA. 
Graptolites—The common form of the graptolite-structure is that 
of a narrow band or “stipe,” with a row of “teeth,” z.e., the mouths 
of cells, on one or on both sides. The teeth or serratures are pointed 
or even mucronate in some species, and obtuse in others. Sometimes 
in place of forming a narrow band, the cell-strueture takes a leaf-like 
shape, and at other times it assumes a spiral or convolute form. Spe- 
cimens have also been found, more especially in the Quebec group of 
rocks in the vicinity of Point Levi, in which several stipes cross each 
other or radiate from a common centre, around which there is a thin 
connecting membrane. Our ordinary examples, it is thus evident, 
are merely fragments of the true graptolite-structure ; and as some of 
these occur in branching forms, of which the branches are only toothed 
on one side whilst the main stem is toothed on both margins, it is 
more than probable that the same species has been described in some 
instances under different names. Being entirely confined to the Silu- 
rian strata, tne graptolites are especially interesting and valuable as 
geological test-forms. On this continent they are chiefly characteristic 
of the Lower Silurian division, (see Parr V.) By some authors, the 
forms with serratures on each side of the stipe are described under 
the generic name of Diplograpsus; and those with serratures on one 
side enly, under that of Graptolithus. 
As examples ot Canadian forms,.we may cite at present Graptoli- 
thus Logani, ig. 67. from the base of the Lower Silurian formation; 
Graptolithus (or Diplograpsus) pristis, Fig. 68, with acute or sub- 
mucronate serratures, from the Trenton limestone, Utica Slate, and 
Hudson River group of the same formation; G@. (=Diplograpsus) 
ramosus, with obtuse or somewhat truncated serratures, Fig. 69, from 
the Utica Slate and Hudson River group (Lower Silurian); and G. 
priodon, (=G. clintonensis, Hall) Fig. 70, with reversed serratures, 
from the Clinton and Niagara group of the Upper Silurian series, 
