DESCRIPTIVE REMARKS, XL 
Fosstns oF THE SILURIAN Rocks. 
The succeeding Formations of the next Fossiliferous epoch, the Silu- 
rian, disclose a considerable and rapidly increasing addition to the as- 
semblage of Fossils found in them, especially in those deposits which 
were evidently favourable to their preservation ; Corals, Mollusca, 
Crustacea, and Echinodermata being 
the classes ofanimals represented. We 
do not meet with any remains of a 
higher character than Invertebrata, 
until we arrive at the very uppermost 
beds of the Silurian strata, when we 
get the first evidence of vertebrate ani- 
mals in the remains of fish. 
The extinct order of Crustacea called 
Trilobites, from their being three- 
lobed, are amongst the most important 
of Paleozoic Fossils, especially in the 
lowest division of these strata, the 
Lower Silurian, and the most highly 
organized of the Invertebrata occur- 
ringinthem. To assist in their study, 
an outline sketch of a common Upper 
Silurian species, Phacops caudatus, 
with a definition of the principal parts, 
is shownin the woodcut, fig. 2; alsothe 
under side ofthe head, with thelabrum 
at fig. 3 (on the following page), from 
a specimen of Calymene tuberculosa. 
No trace of feet or swimming appen- 
dages has yet been observed on the un- 
der surface of the Trilobite, although 
both sides of the hard crust of these ani- 
Fig. 2.—Parts of a Trilobite. 
Phacops Caudatus. 
A. Ifead, or Cephalic shield. 
a. Glabella. mals hasbeen found, and their remains 
b, Sides, oe occurin every condition ofrock favour- 
d. Front margin, able to the preservation of such organs, 
e, Eyes. | a = Aneabl 4 Wee 
f, Posterior margin, if sufficiently endurableto have become 
g- Posterior angles, or cheek spines. fossilized. Mr. Salter, therefore, con- 
h- Facial sutures, or divisions of the head. cludes they were destitute of any such 
B. Thorax ; thoracic rings, or segments of the y ey 2 oan 
body. appendages, and that the ventral sur- 
rape CE eeeatO face had not even membranous fect. 
’ Ss. ° 
Pomel : It appears, however, scarcely possible 
eke eae nos siield that they were without locomotive or- 
me MEACEO. gansofsome kind; probably these were 
soft, and unpreservable, like those of some of the Crustacea, and other 
allied marine animals of the present day. Professor Burmeister gives an 
* 
