Amongst the lower forms of animal life (_Protozoa_), we find the Sponges represented by the curious bodies, composed of netted fibres, to which the name of _Protospongia_ has been given (fig. 32, a); and the comparatively gigantic, conical, or cylindrical fossils termed _Archoeocyathus_ by Mr Billings are certainly referable either to the _Foraminifera_ or to the Sponges. The almost total absence of limestones in the formation may be regarded as a sufficient explanation of the fact that the _Foraminifera_ are not more largely and unequivocally represented; though the existence 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 absence of corals, so far as at present known. [Illustration: Fig. 29.--A portion of _Oldhamia antiqua_, Lower Cambrian, Wicklow, Ireland, of the natural size. (After Salter.)] The group of the _Echinodermata_ (Sea-lilies, Sea-urchins, and their allies) is represented by a few forms, which are principally 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, however, appears to occur in the Lower Cambrian. [Illustration: Fig. 30.--Annelide-burrows (_Scolithus linearus_) from the Potsdam Sandstone of Canada, of the natural size. (After Billings.)]