W. D. Lang—Cretaceous Polyzoa of the Family Idmoniide. 129 
CrisinA—RETECAVA. 
A. Zoarium cylindrical or subtriangular or subquadrangular in cross-section, or 
flattened in a plane at right angles to the obverse and reverse faces. 
I. Lateral series quite irregular. 
a. Zoarium with a strong, blunt median keel 
6, Zoarium with a strong, thin median keel : 
II. A few scattered apertures between the somewhat wavy lateral series 
III. Lateral series regular. 
a. Apertures elongate in a proximal-distal direction. 
1. Apertures about four in a series : Sc se She sya 
2. Apertures ten or more in a series 4 ad s0° aie neo 
b. Apertures circular or nearly so. 
1. Reverse face convex. 
a. Apertures one or occasionally two in a series Be Wee “oy 8 
8. Apertures 2—6 in a series. 
a. No decided keel on the obverse face, nor much compressed. 
1. Apertures decreasing in size from the middle of the obverse © 
face outwards ... : ig aie See Ee oe 
2. Apertures all of one size. 
a. Zocecia immersed and boundaries to zocecia not strongly 
marked. 
a. Series of apertures do not reach the middle line of 
obverse face. 
1. Branches connected by cross-bars which bear no 
apertures re doe Bt a aber ics} 
. Branches with no eross-bars. 
a. Distance between series of apertures 5-8 or more 
times the diameter of aperture. 
a. Apertures two in a series, branches take a Zigzag 
onwre 
course; _<.. 9 
b. Apertures 3-4 in a series ; : “those of same series 
distant about half diameter of aperture soo LUO) 
c. Apertures 4—5 in a series; those of same series 
: distant three-quarters diameter of aperture... 11 
B. Distance between series of aera 2-3 diameters 
of apertures ... a6 see goo. 14 
b. ere of apertures meet the middle ine. 
. Apertures of same series distant less than one diameter 
of aperture. 
a. Apertures of different series distant 1-3 diameters of 
aperture 50 13 
B. Apertures of different series distant about three 
diameters of aperture . 14 
y. Apertures of different series distant 4-8 diameters of 
aperture 356 5 116) 
2, Apertures of same series distant one diameter of ‘aperture 16 
8. Zocecia immersed, but boundaries of zocecia are strongly 
marked. 
: Zocecia two only in a series ae Lay son les 
. Zocecia three (occasionally two) in a series ode soo IG) 
3. Zoecia fourin aseries ... 19 
4, Zocecia six in a series (series of apertures ‘do not reach the 
middle line) ras sae als ae a8 aa, AY 
. Zocecia emergent sae 2 
b. A decided keel on obverse face or decidedly compressed. 
1. Apertures of same series distant one or more than one diameter 
of aperture. 
a. Reverse face marked with linear depressions Se Se 2 
me Reverse face marked with reticular depressions _.. 23 
2. Apertures of same series distant less than one diameter ot 
aperture. 
DECADE V.—VOL. IV.—NO. III. 9 
