Fig. 6.—Structures of A. profundus (magnified).
From specimens in Peter Redpath Museum.
(a) Lower acervuline portion. (b) Upper part, with three of the radiating laminæ and section of pores, (c) Portion of lamina, with pores, the calcareous skeleton unshaded.
These are the humblest of all the inhabitants of the sea, presenting very simple, jelly-like bodies with few organs, but sometimes producing complex and beautiful calcareous and siliceous coverings or tests. Animals of this type have been found in the Lower Cambrian, though not in such vast multitudes as in some later formations. There are also in the Cambrian some large, laminated, calcareous bodies (Cryptozoon of Hall), to be noticed more fully below, and which have recently been traced in still lower deposits even below the lowest Cambrian (Figs. [7], [8]). These have some resemblance to the layer-corals or stromatoporæ of the Silurian and Ordovician, which are by many regarded as the skeletons of coral animals of a low type; but the microscopic structure of Cryptozoon rather allies it with some of the larger forms of Protozoa found higher up in the series of formations. We shall have to discuss this later in connection with still older fossils.
Fig. 7.—Cryptozoon proliferum, Hall.
Portion of slab reduced in size. (After Hall.) See also [Fig. 59, p. 237].
Fig. 7a.—Portion of thin section of Cryptozoon proliferum (magnified × 50).
(a) Corneous layers, (a¹) One of these dividing, (b) Intermediate stroma with granules of calcite, dolomite and quartz, traversed by canals.
From a Micro-photograph by Prof. Penhallow.
[To face p. 39.
If now in imagination we cast our tow-net or dredge into the sea of the Lower Cambrian, we may hope to take specimens illustrative of all our six groups of invertebrate animals, and under several of them examples of more than one subordinate group. Of the Crustaceans we might have representatives of four or five ordinal groups, and of the Mollusca as many. These are the two highest and most complicated. In the four lower groups we would naturally have less variety, though it would seem strange, were it not for so many examples in later periods, that the dominant and highest groups should be most developed in regard to the number of their modifications.
