124 W.D. Lang—Cretaceous Polyzoa of the Family Idmoniide. 
d’Orbigny’s figure of the only species of that genus. At the extreme 
proximal end of the zoarium the apertures are in single lateral series, 
l.e. proximally the form is an Zdmonea. 
Without the knowledge of the proximal end of Birtubigera it is 
uncertain whether this form arose directly from Crisina, as Repto- 
fascigera from Idmonea, by doubling the single lateral series of 
apertures, or through intermediate unknown forms from Reptofascigera. 
The following diagram summarizes these suggestions as to the 
origin of the simpler genera :— 
Crisina —-— _ By doubling the single lateral 
By throwing up solid branches. series of apertures. 
Bitubigera. 
[ By throwing up erect solid 
branches, possibly through 
Phalangella. intermediate unknown forms. | 
Semiclausa. By shortening the central axis so ‘ 
By the erection of — that the apertures are arranged 
the zoarium and its radially rather than pinnately, i 
folding to form and the zoarium is discoid rather Reptofuscigera. 
a hollow tube. < than strap-shaped. 
By doubling the single 
bovis an eee series of apertures. 
Idmonea. 
The forms distributed among the remaining genera are not easy to 
group. TZervia, Reticrisina, and Crisina (restricted) are considered by 
Gregory as subgenera of Criscna; and there is no doubt of their close 
relationship. But the species of Crisina and of Retecava are very 
hard to separate, and the latter genus resembles Sudcocava and 
Bicrisina. Pergensella is very isolated, and there is no evidence of its 
origin. 
The following scheme suggests the main lines of development, but 
it does not represent all the facts, for species of intermediate nature 
occur between various genera, showing that probably the genera are 
artificial. Hence the clumsy definitions in the table of genera given 
above. 
Reticrisina, pars. 
By compression of the zoarium in a 
plane perpendicular to that of the 
obverse and reverse faces. 
Sulcocara. 
By the reduction of 
the reverse face. Reticrisina, pars. 
N By reduction of the reverse face. 
i 
By the appearance <— Crisina —> By irregularity of the lateral series 
of depressions be- | when they meet in the middle line 
tween the apertures. of the obverse face, resulting in 
V a single median series. 
By the compression of the By the swelling of the reverse Lervia. 
Zoarium perpendicularly face to form an ‘ axial rod.’ 
to the plane of the Retecava. 
obyerse and reverse faces. 
Birerisina. 
