XVI 
INTKODtrCTION. 
the Cheilostomata, is not true for some Mesozoic Cyclostomata. 
Thus the aperture in Haplocecia and in the Eleids is similar to that 
of the simpler Cheilostomata; while in some Cheilostomata, such 
as Liriozoa, the aperture is “ almost terminal,” * and in Bigemellaria 
it is “ subterminal.” ^ In Liriozoa, in fact, the aperture is as plain 
and simple as in ordinary Cyclostomata. 
Hincks’ ^ definition of Cyclostomata — “ Zooecia tubular, with 
a plain, inoperculate orifice ; marsupial and appendicular organs 
wanting” — is also useless when the Cretaceous fauna is included. 
Of its five statements three are negatiye ; a fourth, the tubular 
nature of the zooecia, is equally true of some Cheilostomata, 
as is shown by Hincks’ own diagnosis of the JEteidae ; and the 
fifth character, the plainness of the orifice, is an assumption that 
can only be tested on well-preserved recent species, and cannot 
be verified in the case of any fossils or of the majority of recent 
museum specimens. 
Three Orders of Bryozoa, including five distinct groups, entered 
the Cretaceous from the Jurassic. 
1. Trepostomata, an Order comprising forms with a massive 
zoarium composed of tubular zooecia. This Order survived from 
the Palaeozoic, and lingered to the Cainozoic. The Trepostomata 
are abundant in the Lower Cretaceous, but they become much 
scarcer and the forms smaller in the Upper Cretaceous, and these 
changes mark the decline in the importance of the Order. 
2. The Cyclostomata are the predominant Bryozoa in the 
Cretaceous, and the Order is represented by three suborders, each 
of which dates from the Jurassic. They are — 
{a) Tubulata, the dominant forms in the Jurassic, which in the 
Cretaceous attained their maximum of number and variety. 
(b) Uactylethrata, an oflPshoot from the Tubulata, in which the 
zoarium is increased in complexity by the presence of dactylethrae. 
{c) Cancellata, in which the walls of the zooecia are perforated 
by cancelli. True cancelli, which are cavities in the walls of the 
zooecia, though of interzooecial origin, have to be distinguished 
from, interspaces between adjacent zooecia. 
^ P. H. MacGillivray. Monogr. Tert. Polyz. Victoria : Trans. Roy. Soc. 
Viet. iv. 1895, p. 6. 
2 Ihid. p. 7. 
3 T. H. Hincks. Brit. Mar. Polyz. 1880, p. 1. 
