THE CAMBRIAN PERIOD. 85 
in some of the Cambrian Trilobites, such as the little Agwost7 
(fig. 31, g), the animal was blind. The lateral portions of the 
Fig. 31.—Cambrian Trilobites: a, Pavadoxrides Bohenticus, reduced in size; 4, Ellif- 
socephalus Hoffi; c, Sao hirsuta; ad, Conocoryphe Sultzeri (all the above, together with 
fig. g, are from the Upper Cambrian or ‘‘ Primordial Zone” of Bohemia); ¢, Head-shield 
of Dikellocephalus Celticus, from the Lingula Flags of Wales; 4, Head-shield of Cozo- 
coryphe Matthewt, from the Upper Cambrian (Acadian Group) of New Brunswick; g, 
A gnostus rex, Bohemia ; 4, Tail-shield of Dikellocephalus Minnesotensis, from the Upper 
Cambrian (Potsdam Sandstone) of Minnesota. (After Barrande, Dawson, Salter, and 
Dale Owen.) 
head-shield are usually separated from the central portion by 
a peculiar line of division (the so-called ‘facial suture”) on 
each side; but this is alsa wanting in some of the Cambrian 
species. The backward angles of the head-shield, also, are 
often prolonged into spines, which sometimes reach a great 
length. Following the head-shield behind, we have a portion 
of the body which is composed of movable segments or ‘“‘body- 
rings,” and which is technically called the ‘‘ thorax.” Ordi- 
narily, this region is strongly trilobed, and each ring consists of 
a central convex portion, and of two flatter side-lobes. The 
number of body-rings in the thorax is very variable (from two 
to twenty-six), but is fixed for the adult forms of each group of 
the Trilobites. The young forms have much fewer rings than 
the full-grown ones ; and it is curious to find that the Cam- 
