ily 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 and
characteristic of these belong to the great family of the Oysters
(_Ostreidoe_). Amo