It is curious that in the line of succession above stated, the beautiful tubulated cell-wall of _Eozoon_ disappears; and this structure seems, after the Laurentian, to be for ever divorced from the great laminated Protozoans. It reappears in the Carboniferous, in certain smaller organisms of the type of the _Nummulites_, or Money-stone Foraminifers, and is continued in this group of smaller and free animals down to the present time. In the Cretaceous and early Tertiary periods, the Foraminifera of different types have been nearly as great rock-builders as they were in the Laurentian. Some of these later rock-builders, however, have belonged to the lower or imperforate group; others to the higher or Rotaline and Nummuline groups; and, as a whole, they have been individually small, making up in numbers what they lacked in size. Probably the conditions for enabling animals of this type rapidly, and on a large scale, to collect calcareous matter, were more favourable in the Laurentian than they have ever been since. [Illustration: FIG. 24.—_Archæocyathus minganensis._ A Primordial Protozoon.—After Billings. _a_, Pores of the inner wall.] In the Siluro-Cambrian age two other forms of gigantic Foraminiferal Protozoans were introduced, widely different from _Eozoon_, and destined apparently not to survive the period in which they appeared. These were _Archæocyathus_, the ancient Cup-corals, and Receptaculites, which may perhaps be called the Sack-corals. Both are quite remote from _Eozoon_ in structure, wanting its complexity in the matter of minute tubules, and having greater regularity and complication on the large scale. _Archæocyathus_ had the form of a hollow inverted cone with double perforated walls, connected by radiating irregular plates, also perforated (Fig. 24). It has been regarded as a sponge, and some species are certainly accompanied with spicules; but these I have ascertained to be merely accidental, and will be referred to in the next chapter. The true structure of _Archæocyathus_ consists of radiating calcareous plates enclosing chambers connected by pores. Archæocyathus came in with the Later Cambrian, and seems to have died out in the Siluro-Cambrian. The only more modern things which at all resemble it are the Foraminifera called _Dactylopora_, which belong to the Tertiary period. [Illustration: FIG. 25.—Receptaculites. Restored.—After Billings. _a_, Aperture. _b_, Inner wall. _c_, Outer wall. _n_, Nucleus, or primary chamber. _v_, Internal cavity.] Receptaculites is a still more complex organism. It has a sack-like form, often attaining a large size, and the double walls are composed of square or rhombic plates, connected with each other by hollow tubes from which proceed canals perforating the plates (Fig. 25). This curious structure is confined to the Siluro-Cambrian, and is so dissimilar from modern forms that its affinities have been subject to grave doubts.