82 HISTORICAL PALZZONTOLOGY. 
drical fossils termed Avcheocyathus by Mr Billings are certainly 
referable either to the Foraminifera or to the Sponges. ‘The 
almost total absence of lime- 
WF stones in the formation may 
Y be regarded as a sufficient ex- 
planation of tne fact that the 
Foraminifera are not more 
largely and unequivocally re- 
presented ; though the exist- 
ence of greensands in the 
Cambrian beds of Wisconsin 
and Tennessee may be taken 
as an indication that this class 
of animals was by no means 
wholly wanting. The same 
fact may explain the total ab- 
sence of corals, so far as at 
present known. 
The group of the Lchinoder- 
Fig. 29.—A portion of O/dhamia an- mata (Sea-lilies, Sea-urchins, 
tigua, Lower Cambrian, Wicklow, Ire- : : ° 
land, of the natural size. (After Salter.) | and their allies) is represented 
by a few forms, which are prin- 
cipally of interest as being the earliest-known examples of the 
class. It is also worthy of note that these precursors of a 
group which subsequently attains such geological importance, 
are referable to no less than three distinct orders—the Crinoids 
or Sea-lilies, represented by a species of Dendrocrinus; the 
Cystideans by Protocystites; and the Star-fishes by Padasterina 
and some other forms. Only the last of these groups, how- 
ever, appears to occur in the Lower Cambrian. 
The Ringed-worms (Annelida), if rightly credited with all 
the remains usually referred to them, appear to have swarmed 
in the Cambrian seas. Being soft-bodied, we do not find the 
actual worms themselves in the fossil condition, but we have, 
nevertheless, abundant traces of their existence. In some 
cases we find vertical burrows of greater or less depth, often 
expanded towards their apertures, in which the worm must 
have actually lived (fig. 30), as various species do at the pre- 
sent day. In these cases, the tube must have been rendered 
more or less permanent by receiving a coating of mucus, or 
perhaps a genuine membranous secretion, from the body of 
the animal, and it may be found quite empty, or occupied by 
a cast of sand or mud. Of this nature are the burrows which 
have been described under the names of Scolithus and Scoleco- 
derma, and probably the Aistioderma of the Lower Cambrian 
