»34 Ulrich on Genera of (Bryozoa. 
ary set. These, again, in like manner articulate with still more 
slender tertiary segments, forming at the same time jointed 
parallel branches of the primary series of segments. 
Excepting in the Trenton beds near Ottawa, Canada, the 
segments of Arthrocletiia are rarely found still in connection. 
As the isolated segments closely resemble those of Helopora 
they might readly be mistaken for a species of that genus. 
They might even be supposed to represent two or three species, 
since the secondary and tertiary elements are considerably 
smaller than the primary one. This fact furnishes another 
point of difference from Helopora^ since in that genus all the 
segments are approximately equal. The main difference, 
however, is found in the two or more sockets that occur on the 
sides of the primary and secondary segments of Arthroclema. 
In the typical species of the new genus Netnatopora the mi- 
nute characters and zooecial arrangement closely resemble those 
of Arthrostylus^ orily the zooecia open on all sides of the branch- 
ing stems. The type and several other species are from the 
Trenton limestone; two species occur in the Cincinnati group, 
several in the Anticosti, and two or more in the Niagara. 
Hall's Trematopora minuta belongs here. 
In the above short sketch I have endeavored to give the ' 
student a fair idea of the distinctive characters of the several 
genera comprised in the Arthrostylidce. On account of the 
small size of the segments the species will probably continue to 
be overlooked by the ordinary collector, but he who takes an 
interest in palaeozoic bryozoa should not fail to search for them, 
as I can assure him he will find them not only an interesting 
group, but also attractive objects under the microscope. Thin- 
sections are easily prepared of most of them by simply taking ' 
slices of the rock holding them; and none are too small to be 
thus studied. All of my sections of them have proved more 
or less instructive and not a few make really handsome micro- 
objects. 
and slightly concave centrally, the lower moderately convex. Zocecia 
arranged in quincunx; measuring lengthwise seventeen in 5 mm.; diag- 
onally, four in i m. Apertures direct, oval, with a very narrow peri- 
stome, set into a rhomboidal or hexagonal concave area. A rather strong 
acanthopore generally occupies the space between the ends of the 
zooecia apertures. 
