Coming to the Annulose Animals of the Cretaceous period, there is little special to remark.  The Crustaceans belong for the most part to the highly-organised groups of the Lobsters and the Crabs (the Macrurous and Brachyurous Decapods); but there are also numerous little Ostracodes, especially in the fresh-water strata of the Wealden.  It should further be noted that there occurs here a great development of the singular Crustaceous family of the Barnacles (Lepadidoe), whilst the allied family of the equally singular Acorn-shells (Balanidoe) is feebly represented as well.

[Illustration:  Fig. 191.—­Galerites albogalerus, viewed from below, from the side, and from above.  White Chalk.]

[Illustration:  Fig. 192.—­Discoidea cylindrica; under, side, and upper aspect.  Upper Greensand.]

Passing on to the Mollusca, the class of the Sea-mats and Sea-mosses (Polyzoa) is immensely developed in the Cretaceous period, nearly two hundred species being known to occur in the Chalk.  Most of the Cretaceous forms belong to the family of the Escharidoe, the genera Eschara and Escharina (fig. 193) being particularly well represented.  Most of the Cretaceous Polyzoans are of small size, but some attain considerable dimensions, and many simulate Corals in their general form and appearance.

The Lamp-shells (Brachiopods) have now reached a further stage of the progressive decline, which they have been undergoing ever since the close of the Palaeozoic period.  Though individually not rare, especially in certain minor subdivisions of the series, the number of generic types has now become distinctly diminished, the principal forms belonging to the genera Terebratula, Terebratella (fig. 194), Terebratulina, Rhynchonella, and Crania (fig. 195).  In the last mentioned of these, the shell is attached to foreign bodies by the substance of one of the valves (the ventral), whilst the other or free valve is more or less limpet-shaped.  All the above-mentioned genera are in existence at the present day; and one species—­namely, Terebratulina striata—­appears to be undistinguishable from one now living—­the Terebratulina caputserpentis.

[Illustration:  Fig. 193.—­A small fragment of Escharina Oceani, of the natural size; and a portion of the same enlarged.  Upper Greensand.]

[Illustration:  Fig. 194.—­Terebratella Astieriana.  Gault.]

Whilst the Lamp-shells are slowly declining, the Bivalves (Limellibranchs) are greatly developed, and are amongst the most abundant and characteristic fossils of the Cretaceous period.  In the great river-deposit of the Wealden, the Bivalves are forms proper to fresh water, belonging to the existing River-mussels (Unio), Cyrena and Cyclas; but most of the Cretaceous Lamellibranchs are marine.  Some of the most abundant