28 A POPULAR EXPOSITION OF THE 
permitted the trilobites to bring the under parts of the caudal and 
head-shields together, both for the protection of the soft or unde- 
fended parts of the body, and also, in all probability, to enable the 
creature to sink with greater rapidity into deeper water during 
moments of danger or alarm. Specimens in this ‘rolled up” con- 
dition are of very common occurrence (see fig. 143 a, and 144). 
The shell covering of the pygidium or “tail” (P in fig. 138), 
consists of a single or entire piece: or rather, perhaps, of various 
consolidated segments. It is very generally met with detached from 
the other portions of the body. Its outline is either rounded, pointed, 
or digitated ; and it sometimes terminates in a long spine, or exhibits 
several spinous processes. In some genera it is very small, whilst in 
others it equals the head-shield in size. 
The more important genera and species of Trilobites, occurring in 
Canadian rocks, are enumerated below: 
Trinuclers.—Head-shield surrounded by a perforated border ; gla- 
bella, globose and strongly pronounced ; eyes, 
wanting. Six body-rings. Caudal-shield of 
moderate size. 7’. concentricus (fig. 139), of 
the Trenton and Hudson River Groups, is our 
only species; but examples of this (in a more 
or less fragmentary state) are common. When 
perfect, the corners of the head-shield termi- 
nate in horns, and a spine projects backwards 
from the base of the glabella. Average length 
one to two inches. AB 
Asaphus.— Head, thorax, and pygidium, of about equal size. 
Glabella smooth or slightly furrowed, and not much raised. Eyes 
tolerably near together. Hypostoma forked. Body-rings, eight in 
number. Our two most common species comprise A. platycephalus, 
formerly called Isoteles gigas (fig. 140), with rounded head-angles 
and nearly smooth pygidium, chiefly from the Trenton Group ; and 
A, Canadensis (Fig. 141), with head-angles terminating in points, 
and with furrowed pygidium, from the Utica Slate deposits. Frag- 
ments of this latter form, and sometimes entire specimens, occur in 
great abundance at Collingwood and at Whitby (see Canadian Journals 
Vol. III., p. 230). The forked hypostoma is shewn at a in the above 
figures. Another species, A. megistos, with smooth pygidium and 
