BRYOZOA. 161 
Trigonodictya, 
twice as large as those in the central rows. When the surface is weathered the 
zocecial apertures are larger than normal, and their longitudinal arrangement less 
obvious, the interspaces rounded, and without the series of minute papille# that are 
always present when the surface is well preserved. 
In considering the internal characters it should be borne in mind that but few 
tangential sections are at all likely to be made that will show the structure as fully 
and clearly as in fig. 12 (plate IX). The section from which this drawing was made 
is an exceptionally good one, having been prepared from a fragment in an unusually 
good state of preservation; so that it shows the structure just beneath the surface in 
a very satisfactory manner. At the sides of the figure, the left-hand one especially, 
the horizontal median tubuli are represented, and a short distance from the edge we 
see how the vertical series of these tubuli arise out of the horizontal set.* At 
a deeper level than any shown in the figure, the zocecia are larger and rounder, and 
the interspaces proportionally narrower, and, with the exception of a dark line run- 
ning longitudinally between the rows of zocecia, generally appear structureless. A 
little deeper and a few irregular lines, representing the walls of interstitial vesicles, 
may be noted in the interspaces. 
Transverse sections show that in the regularly developed triangular branches 
each is divisible into three subequal triangular parts, bounded by a mesial line from 
which the zocecial tubes of each part proceed to their respective external faces. 
New angles and faces are produced by raising one of the plate-like longitudinal 
walls until it has assumed the characters of a mesial plate. 
The zoarial features of this species are so strikingly different from all known 
Lower Silurian Bryozoa that comparisons are unnecessary. For remarks on the 
Clinton group species.of the genus, see under the generic description. 
Formation and locality—A pparently restricted to the upper third (Phylloporina horizon) of the 
Trenton shales, near Cannon Falls, Minnesota. : 
Mus, Reg. No. 5952. 
Family PTILODICTYONID, Zittel. 
For a description and remarks on this important family, the reader is referred 
to my recent work in the eighth volume of the reports of the geological survey of 
Illinois, pp. 348 and 390. 
Five genera of this family are represented in the Trenton shales of Minnesota, 
and, so far as known, the species here described of each are the earliest existences 
*This is not only an interesting fact, but, as are all that relate to the intercommunication of the zooids, also one of 
great importance morphologically, 
a (Bi 
