XiV DESCRIPTIVE REMARKS. 
ideal figure of the under surface of a Trilobite, with such an arrangement 
of soft parts, but which, although interesting as the opinion of so emi- 
nent an authority, is necessarily extremely conjectural.* 
With respect to their relations with exist- 
ing Crustacea, they are believed to occupy 
a distinct group by themselves, present- 
ing, however, such a resemblance to some 
recent forms of the Peecilopoda and Phyl- 
lopoda, as to induce some anthors to con- 
sider them to be intermediate, or closely al- 
lied to those orders of the Entomostraca or 
lower division of the Crustacea. 
The range of this important group, as 
shown by Mr. Salter,t is from the lowest 
beds of the Paleozoic series, the Lingula 
Flags, to the upper, or Carboniferous For- 
Fig. 3. 
Calymene tuberculosa, showing un- 
Sues mide vot the Head. mation, its greatest abundance being in the 
a. ostral shield. . . 
%. Rostral suture, | Llandeilo and Caradoc Formations of the 
c. Part of the facial suture. S 2 3 * CaS 
Gutaepin of head. Lower Silurian; gradually lessening in im- 
e. Labrum, epistome, or hypostome. portance to the upper part of the Carbonife- 
f. Ends of pleuree. 
g. Tail, or pygidium. rous series, when they became extinct. 
Fossits oF THE LowER SitvuR1AN Rocks. 
To the labours of the Rev. Professor Sedgwick, and Sir Roderick 
Murchison, we are mainly indebted for the elucidation of the great 
series of strata known as the Lower Paleozoic Rocks. 
This series of rocks, for the most part consisting of calcareous and 
argillaceous deposits, alternating with limestones, sandstones, and crys- 
talline slates, are principally developed in North and South Wales, 
Shropshire, and the North-west of England; also to a considerable ex- 
tent in the South of Scotland, and distributed over several counties in 
various parts of Ireland. 
The principal and most of the minor divisions in the order of suc- . 
cession of Silurian strata adopted by the Geological Survey (and which 
we have followed) are, as Professor Ramsay informs us, almost identi- 
cal with those published in 1889 by Sir R. Murchison in the Silurian 
System ; ‘‘the chief differences consisting in additions to our knowledge, 
due either to the discovery since that period of new and subordinate di- 
visions, or to the betfer understanding of the precise stratigraphical re- 
* Organisation der Trilobiten, by Dr. H. Burmeister; pl. vi., fig. 8. 
+ British Trilobites, Paleontographical Society (1864), pp. 6, 7. 
{ See Table of Fossiliferous Strata, ante, pp. 7, &e. 
