BEYOZOA. 161 



Trigonodlctya, 



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 papillse that are 

 always present when the surface is well preserved. 



In considering the internal characters it should be borne in mind that but few 

 tan^ntial 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 zooecia are larger and rounder, and 

 the interspaces proportionally narrower, and, with the exception of a dark line run- 

 ning longitudinally between the rows of zooecia, 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.— Ap^SiTentiy restricted to the upper third (Phylloporina horizon) of the 

 Trenton shales, near Cannon Falls, Minnesota. 



Mus. Beg. No. 5952. 



Family PTILODICTTONID^, 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 



*Thls is not only an interesting fact, but, as are all that relate to the Intercommunication of the zooids, also one of 



great Importance morphologically. 

 -11 



