.W.D. Lang—Cretaceous Polyzoa of the Family Idmonide. 123: 
C. Zoarium erect, of solid branches. 
I. No unpaired median nor multiple median series of apertures, 
a. Apertures in double lateral series ... a Bi. : 
(Only Cretaceous species, B. compressa, Marsson, 1887, 
Bry. Rug. : Pal. Abh., vol. iv, pl. iu, fig. 1.) 
b. Apertures in single lateral series. 
1. Apertures circular or tear-shaped. In the former case 
longitudinal grooves, slots, or circular depressions 
are present on the zoarium; in the latter case the 
reverse face is very narrow. 
a. Zoarium very compressed in a plane at right angles a) 
to that of the obverse and reverse faces ; reverse face 
well developed Sve 2 a2 aoe 556 ... Bicrisina. 
8. Zoarium not very compressed ; reverse face very narrow Swleocava. 
2. Apertures circular (tear - shaped in two species of 
Relecava, but in both the reverse face is well 
developed). If longitudinal grooves or depressions 
are present on the zoarium, it is never very com- 
pressed in a plane at right angles to that of the © 
obverse and reverse faces (as in Bierisina), nor is the 
reverse face reduced (as in Svlcocava). 
a. Branches very compressed in a plane at right angles 
to that of the obverse and reverse faces ; or if not so . 
compressed the reverse face is very narrow... ... Reticrisina. 
B. Branches cylindrical or moderately compressed ; reverse 
face well developed. 
a. An ‘axial rod’ more or less developed on the 
reverse side 50 36 oe are ... Retecwa. 
b. No ‘ axial rod’ oF ue ae Sc ... Crisina, 
II. An unpaired median series of apertures; lateral series simple Tervia. 
III. A multiple median series of apertures ; iateral series multiple Pergenselia. 
(Only species— 
Idmonea geniculata, von Hagenow, 1851: Bry. Maastr. 
Kr., pl. iii, fig. 5. . 
Retepora clathrata, pars, Goldfuss, 1827: Petrefact. 
Germ., vol. i, pl. ix, figs. 12e—f. 
Pergensella geniculata, Gregory, 1899: Brit. Mus. Cat. 
Cret. Polyzoa, p. 210, figs. 19-20.) 
Until the forms consisting of solid branches are reached, the 
development of the family appears fairly simple. The genus Semz- 
clausa, consisting of hollow tubes, probably arose, like the hollow 
Diastoporas (such as D. ramosissima, d’Orbigny), by the folding of an 
erect unilamilar zoarium. Such a form has not yet been found; but 
it only needs the erection of the distal end of an /dmonea to produce 
it. Until the proximal ends of d’Orbigny’s two species of Semiclausa 
are found nothing definite can be known of their origin. For it is. 
conceivable that they may be encrusting forms which have grown on 
some perishable cylindrical base, such as a stem of seaweed, the 
decomposition of which has left the encrusting Polyzoan in the form 
of a hollow tube. 
Gregory has produced evidence of the origin of Crisina in the shape 
of a discoid base of this genus resembling Phalangella.' This is 
exactly comparable with the specimen he figures of an Extalophora 
arising directly from a Berenicea-like base.* 
The origin of Reptofascigera from Idmonea is clearly shown in 
Bitubigera. 
1 J. W. Gregory: Brit. Mus. Cat. Cret. Bryozoa, vol. i (1899), p. 159, fig. 11. 
2 J. W. Gregory: loc. cit., p. 217, fig. 26. 
