AND DISTRIBUTION OF TRILOBITES. 269 
II. On the Genera of the Trilobites. 
In point of their systematic position, I regard the Trilobites 
as constituting a peculiar order of Crustacea, at present perfectly 
extinct, and which certainly connects the two divisions of the 
Malacostraca and Entomostraca, but approximating nearer to 
the latter than to the former. They are related to the former 
by their calcareous crust-like shell, and by their not possessing 
simple eyes in conjunction with compound eyes. The wood- 
lice have the greatest similarity to them among the divisions 
of the shelled Crustacea. Limulus and Phyllopoda are the two 
orders among the Entomostraca, to which they are united by 
the nearest characters of affinity. They correspond with these 
two orders in point of form and size of the clypeoid head; and, 
moreover, with the latter as regards the rings of the thorax, 
which are moveable, and vary in point of number. The soft 
texture of the under side of their body, and the circumstance 
that no one has as yet succeeded in detecting unquestionable 
vestiges of legs, are in favour of the supposition that they must 
have corresponded likewise in the structure of their feet with 
Apus and Branchipus. Nor have we as yet any positive know- 
ledge with regard to palpi. Portlock has certainly discovered 
an articulated member close to the forehead of Nuttainia hi- 
bernica, which he compares to an antenna; but this discovery 
is of too isolated a nature, and requires further confirmation. 
The constant sutures of the cephalic shield, the so-termed facial 
sutures, are wholly peculiar to the Trilobites. 
In the following synopsis I have classified all the genera of 
Trilobites into five groups or families, which may be again sub- 
divided into two sections. 
Section I. With a caudal shield situated behind the articulated 
thorax, and formed of perfect rings, but which are more or less 
coherent. 
Family I. Trilobites with granular facetted eyes ——The eye has 
a facetted horny membrane, the facets being visible even to the 
naked eye. Glabella distinct, frequently strongly arched, gene- 
_ rally broader anteriorly, clavate, more rarely cylindrical, some- 
times rather narrower anteriorly, lobed at each side by four fur- 
rows, the anterior odd lobe (frons) is largest. This frontal part 
is very large in some, of a more or less trapezoidal shape, and 
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