82 



HISTORICAL PALAEONTOLOGY. 



drical fossils termed Archceocyathus by Mr Billings are certainly 

 referable either to the Foraminifera or to the Sponges. The 

 almost total absence of lime- 

 stones in the formation may 

 be regarded as a sufficient ex- 

 planation of the fact that the 

 Foraminifera are not more 

 largely and unequivocally re- 

 presented ; though the exist- 

 ence of greensands in the 

 Cambrian beds of Wisconsin 



Fig. 29. A portion of Oldhamiii ti'i- 

 tiqna, Lower Cambrian, Wick^ow, Ire- 

 land, of the natural size. (After Saltcr.) 



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 Echinoder- 

 mata (Sea- lilies, Sea-urchins, 

 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 Palasterina 

 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 Histiodenna of the Lower Cambrian 



