Ulrich on Genera of (Bryozoa. 231 
while in the latter the zooecia form two leaves grown together 
back to back, in the former they are arranged radially so as to 
form subcylindrical or club-shaped stems. This mode of ar- 
rangement of itself would cause the primitive portion of the 
zooecia of the larger forms of the first family to incline to be- 
come tubular. The reason is obvious, since nearly all the zooecia 
originate at or near the center of the zoarium and being forced 
to adapt themselves to the distance between the axis and the 
outer surface, are necessarily longer than in such flat forms like 
Stictopora in which the rapid development of the cells is not 
retarded by lack of space. 
The resemblances above pointed out sufficiently show the 
position of the family to be near the Stictoporidce^ while their 
relation on the other side again should be sought for among the 
Rhabdo7neso7itidce rather than the Callariidce^ the succession 
of the forms in geologic ages being an important circumstance 
greatly favoring the former. There is, however, a wide struct- 
ural gap between both those families and the Ai-throstylida;^ so 
that we are at present justified in claiming the latter as a dis- 
tinctively Silurian type of which no representatives are yet 
known in succeeding ages. 
The jointed character of the zoarium is the most conspicuous 
as well as the most important feature of the family, being well 
shown in all the genera excepting Ne?7iatofora^ a new genus 
in which the zoarium (very much as in J^iilodiciya, proper) artic- 
ulates at the basal extremity only, and above this base forms a 
continuous dichotomously branching slender stem. In Arthros- 
tylus Ul., Helopora Hall, and Sceptropora^ the segments are 
simple and terminally joined together, each segment giving rise 
to one or two succeeding joints. The first of these genera is 
distinguished by having one of the sides non-celluliferous and 
simply striated longitudinally. In the second the segments are 
subcylindrical with zooecia apertures on all sides, while in the 
third the zoarium is non-celluliferous below and spreads rapidly 
above. 
The typical species of Helopora is Hall's U. fragilis^ a very 
abundant and characteristic fossil of the Clinton group of Can- 
— ■ — — > 
^ This genus together with a number of species will be described in vol. 
viii, Illinois geological survey, now in press. 
