SUB-ORDER C CRYPTOSTOMATA 279 
Chilostomata, which have the same tubular prolongation of the aperture. Thus, the recent 
Adeonella atlantica, Busk, has not only a vestibule, but hemisepta as well. Hemisepta are 
never found in the Cyclostomata and Trepostomata, but are a very common feature of the 
Cryptostomata. They occur at the bottom of the vestibule, and doubtless served as supports 
for the movable operculum. 
Family 1. Ptilodictyonidae. Ulrich. 
Zoaria bifoliate, composed of two layers of zowcia grown together back to back, con- 
tinuous or jointed, and forming leaf-like expansions, or compressed, branching, or tnosculating 
stems. Mesial plates without median tubuli; hemisepta usually present. Inner orifice 
generally semi-elliptical, the outer more rownded, usually ovate, and surrounded by either a 
sloping area or a ring-like peristome. Vestibules separated by thick walls. Ordovician to 
Sub-Carboniferous. 
Ptilodictya, Lonsd. (Heterodictya, Nich.) Zoaria simple, unbranched, lanceolate, 
or falciform, with a small basal expansion. In the young condition the zoarium 
consists of longitudinally arranged, narrow, oblong-quadrate zoccia, new zocecia, of 
different width and arrangement, being added subsequently on each side. In the 
vestibular or outer region the walls are more or less thickened, solid, and with a 
double row of exceedingly minute dots. Ordovician to Devonian. 
Escharopora, Hall (Nicholsonia, Waag. and Wentz). Distinguished from the last 
chiefly by the arrangement of the zoccial apertures, which are in diagonally inter- 
secting series, and by the hexagonal sloping area which surrounds the external orifice. 
Ordovician. 
Clathropora, Hall. Zoaria composed of anastomosing branches forming a regular 
network with round fenestrules, terminating below in a pointed, articulating base. 
Zoccial apertures usually sub-quadrate, arranged longitudinally. Ordovician to 
Devonian. 
Phaenopora, Hall. Zoaria as in Ptilodictya, except that there are two mesopores 
in each interspace between the ends of the zocecial apertures. Ordovician and 
Silurian. 
Arthropora, Ulrich. Zoaria bushy, spreading in a plane, composed of numerous 
equal segments. Zocecial apertures elliptical, surrounded by a delicate peristome. 
Interspaces with one or more threadlike ridges, variously disposed, and with a row 
of minute papillae. Ordovician. 
Graptodictya, Ptilotrypa, Ulrich; Ordovician. Taeniodictya, Ulrich ; Devonian 
and Sub-Carboniferous. 
Stictotrypa, Ulrich. Branches not jointed at the base. Zoccial apertures 
circular or elliptical, with a distinct, evenly elevated peristome. Ordovician and 
Silurian. 
Stictoporella, Ulrich. Zoaria variously formed, with elliptical apertures placed at 
‘the bottom of a sloping area. Thick-walled, untabulated mesopores occur between 
the zocecial apertures and line the zoarial margins. Ordovician to Sub-Carboniferous. 
Intrapora, Coscinella, Hall. Devonian. 
Family 2. Rhinidictyonidae. Ulrich. 
Zoaria bifoliate, continuous or jointed, forming compressed branches or leaf-like expan- 
sions ; occasionally trifoliate. Primitive cells sub-quadrate, arranged longitudinally. Both 
primitive and superficial apertures elliptical or sub-circular, sometimes a little truncated 
posteriorly. Inferior hemiseptum and lunarium wanting. Median tubuli present between 
the median laminae, and between the longitudinal rows of zowcial tubes. Mesopores absent, 
but vesicular tissue often present. Chiefly Ordovician. 
Rhinidictya, Ulrich (Stictopora, Ulrich non Hall). Zoaria composed of narrow, 
