76 W. D. Lang—Caleiwm Carbonate and Evolution. 
carbonate into the elaboration of secondary apertures. The Cheilo- 
stomes do the same with marked (temporary) success. But this is 
only one of their methods. The most hopeful of the Cheilostomes 
are those which have a skeleton of chitin only. When once calcium 
carbonate has begun to be deposited the whole lineage is doomed to 
a more or less stereotyped sequence of calcification until, in the end, 
it becomes extinguished under its superfluity of skeleton. For 
obvious reasons it is only these doomed lineages that we have to 
deal with as fossils; and the evolution of the Cretaceous Cheilostomes, 
as far as we can have any knowledge of it, is the story of the 
progressive elaboration of the calcareous skeleton until this becomes 
secondarily simple by the blotting out of its complexity under 
secondary deposits. 
The simplest Cretaceous Cheilostomes—‘ Membranimorphs ’—have 
calcareous skeletons with no intraterminal front walls.’ Senility in 
an individual results in the complete calcification of the intraterminal 
front wall and the consequent death of the zooid, since even the 
orifice is sealed up. Before this takes place, however, superfluous 
calcium carbonate, in some forms, is deposited as terminal spines.’ 
The general phylogenetic future of some of these, namely those in 
which the terminal spines, arching over, fuse with their opposing 
and lateral neighbours — ‘ Cribrimorphs ’— can be predicted with 
certainty, though the details vary in every lineage. Further calcium 
carbonate is laid down in connection with (1) the spines that form 
the intraterminal front wall—the coste ; (2) those that surround the 
aperture—the ‘apertural spines’ (considered with the first pair of 
cost, those which bound the aperture proximally, and, fused, form 
the ‘apertural bar’); and (3) the extraterminal front wall, finally 
filling up the interzocecial angles; and the process may take place in 
one or more of these directions simultaneously in any lineage. The 
first method results (with various modifications in various lineages) 
in a solid, arched, intraterminal front wall; the second (with various 
interfusions of the apertural spines and apertural bar, often complicated 
by one or more pairs of avicularia entering into the structure) in a 
secondary aperture; the third, again often complicated with avicularia, 
in a generalimmersion of the zocecia beneath interzoecial walls. The 
secondary aperture may be prolonged after the manner of a tubular 
fossil (an interesting case of a primitive method revived); but in 
some lineages the rims of the secondary apertures spread, especially 
proximally, and, meeting with their neighbours, fuse to form 
a secondary front wall (lamina peristomica* of Jullien) above the | 
general zoarial level. Such forms—‘ Steginomorphs ’—though be- 
longing to various lineages, are generally placed in d’Orbigny’s genus 
Steginopora, which, thus used, is seen to be a stage in evolution instead 
of a genus founded on d’Orbigny’s genotypes. It is inconceivable 
that Steginomorphs can have any evolutionary future. In extreme 
cases they present externally a thick crust of calcium carbonate 
pierced here and there by holes representing apertures and avicularia, 
that tend to become smaller and more choked as more calcium 
1 For these terms see Lang, 1914, GroL. MaG., Dec. VI, Vol. I, p. 6. 
2 Jullien, 1886, Bull. Soc. Zool. France, Vol. 11, p. 609. 
