26 A POPULAR EXPOSITION OF THE 
Cyproids, or Bivalve Entomostracans.—The crustaceans of this 
order are more or less minute forms, partly inhabitants of the sea, or 
of brine solutions, and partly of fresh water. The existing marine 
types belong mostly to the genus Cythera or 
Cytherina: the others to the genus Cypris- 
In each, the form is closely alike; and in fossil 
species the one is scarcely to be distinguished 
from the other, except by its associated fossils. 
In living forms, the minute animal is seen to 
possess a delicate bivalve shell, with curious 
tufted feet and antennee, which it projects 
beyond the shell when swimming. These Fig. 187. 
5 Z a@. = Magnified Specimen. 
little crustaceans occur in rocks of all ages, 
and much resemble, in the fossil state, small scattered grains or seeds 
(fig. 137). The shell is frequently brown and lustrous, and usually 
oval or semi-circular in shape. Canadian genera (Organic Remains, 
Decade IV.,) have been referred to Beyrichia, Leperdita, &c., but 
their characters are quite microscopic and more or less indistinct. 
Trilobites.—This order is entirely extinct. Its representatives— 
evidently marine types—are confined to the lower and middle portion 
of the Paleeozoic series; or range, in other words, from the earliest 
fussiliferous rocks into the base of the great Carboniferous formation. 
Above the deposits of the latter geological horizon, not a trace of a 
trilobite has been discovered. The nearest existing type to this extinct 
group, appears to be the Limulus, or “ King-crab’’ ~a form which 
must be familiar to all who have visited the New England coast. 
The shelly covering of the back, with a portion of that which pro- 
tected the under side of the head, are the only 
parts of these crustaceans which have been pre- 
served to us. The back (see fig. 138) consists of 
three principal parts: the Buckler or Head- 
shield, H; the Body or Thorax, T; and the 
Pygidium or Caudal-shield, P. The shell, 
moreover, in most instances, is strongly tri-lobed 
by two longitudinal furrows, as shewn in the 
figure. From this character the order derives 
its name. 
In the centre of the head-shield there is usu- 
ally a distinctly raised portion (= Gin fig. 138) 
