230 Ulrich on Genera of (Bryozoa. > 
The systematic position of Sceptropora is clearly near Helo- 
pora in the family Arthrostylidae} This family especially in 
its most typical members, reminds us of the Cellariidce^ yet, 
beyond the jointed zoarium and a general external resemblance, 
but little can be brought showing true relationship. On the 
contrary, in the form and ininute structure of the zooecia, they 
agree closely with the Stictoporidae.- One of the most marked 
peculiarities of that family is the presence of exceedingly 
minute tubes between the median laminae. In the Arthro- 
stylidae the zocecia are nearly always arranged in a radial 
manner about a central axis. Here a minute vertical tube 
has been detected in exceptionally preserved examples of 
several species, and in at least one species, two such tubes.. 
In Arthrostylus tenuis^ in which the zocrcia open on only 
three sides of the subquadrangular segment, the fourth and 
widest side being simply striated longitudinally, the minute axial 
tube is situated at the point of union between the proxinal ends 
of the three rows of zooecia. This axial tube, or tubes, I regard 
as homologous with the "median tubuli" of the Stictoporidae. 
Another feature common to members of that family, and many 
of Arthrostylidae^ is a row of minute vertical tubuli, which oc- 
cur, particularly in the outer portion of the zoarium, between 
the rows of zooecia, and appear at the surface as minute granu- 
les on the dividing ridges between the rows of superficial aper- 
tures. Precisely the same kind of structures are shown in Scep- 
tropora fac7ila (see fig. i, a) and less well in section of Helopora 
fragilis (fig. 2. c). The primitive or deeper portion of the 
zocecia is, of course, different in the two families, it being wedge 
shaped and generally longer in the Arthrostylidae than in the 
Stictoporidae. These differences, however, are accounted for 
by the different form of zoarium characterizing them. Thus^ 
1 The name Arthrostylus is proposed instead of my Arthronema ("Am. 
Pal. Bry." Jour. Cin. Soc. Nat. Hist. vol. v, p. 160) which was preoccupied 
by Eschscholtz for a genus of Colpodea. The genus being the type of the 
family Arthroncmidae, it seems best to change the family name as well. 
2 This family as now understood and defined by me in the forthcoming 
vol.viiiof the Illinois geological survey, comprises the following genera: 
Stictopora Hall, 1847 not 1887, (based on 5'. fenestrata Hall) Dkratiopora 
Ul., Pachydictya Ul., PhyUodictya Ul., Eurydictya^ n. g'eti : and Euspilopora 
