BRYOZOA. 147 
Pachydictya ] 
the Anticosti group; crassa (Hall), bifurcata (Van Cleve), emaciata (Foerste), farctus 
(Foerste), and rudis (Foerste), from the Clinton, and scituw/a (Hall) from the Niagara.” 
In placing Pachydictya under the Rhinidictyonide I follow the course adopted in 
my 1882 work on the “American Paleozoic Bryozoa,” (Jour. Cin. Soc. Nat. Hist., vol. v), 
and more recently in the eighth volume of the reports of the Geological Survey of 
Illinois, published in 1890. I have always had some doubt as to the strict propriety 
of the arrangement, and the chief reason for its continance in the last work is found 
in the fact that the genus agrees with Rhinidictya and all true genera of the family 
in having “median tubuli.” Now that I am employing the classification for the 
third time, it seems desirable to publish at the same time some account of my objec- 
tions. At first I thought some of proposing a new family for Pachydictya and the 
new genus T’rigonodictya, but was restrained from doing so by the fear that I could 
not, in the present state of our knowledge, satisfactorily establish the distinctness of 
the new family from the Rhinidictyonide. The difficulties are encountered when we 
attempt to draw sharp lines between certain species of Pachydictya on the one side, ° 
and Phyllodictya and Ewrydictya on the other. Had I made the presence or absence 
of diaphragms the test, I would very likely have struck the popular chord, but as I 
know that test to be unreliable only too often when applied to groups of high rank, 
I could not employ it before knowing more of its value in this particular case. 
The suggested removal from the Rhinidictyonide is not caused through any 
depreciation in the value of the character mentioned (median tubuli), but is founded 
upon a better appreciation of certain features wherein Pachydictya and Trigonodictya, 
and in a lesser degree also Phyllodictya, differ from the more typical members of the 
family : Rhinidictya, Dicranopora, Goniotrypa, and Eurydictya. In all of the latter 
the primitive or prostrate portion of the zocecial tube is of an oblong-quadrate or 
rhomboidal shape, the thin wall of adjacent cells being, moreover, in contact with 
each other on all sides. Nor are interstitial vesicles or mesopores present in any of 
them with the single exception of Hurydictya multipora (? Hall’s sp.). Diaphragms, 
also, are very unusual, while a more or less well developed hemiseptum is common. 
Finally, the interspaces, as shown in tangential sections, continue uninterruptedly 
from zoccial cavity to cavity, there being no sharply defined ring-like wall around 
the latter. 4 
In Pachydictya, Trigonodictya and Phyllodictya, however, the hemisepta are never 
present, but complete diaphragms seem to have been developed in all examples old 
enough to have them. Tangential sections bring out peculiarities fully as striking 
and important, but their statement should be premised with the admission that some 
of them are but illy developed, possibly quite unrecognizable, in some of the species. 
*T am convinced that several, perhaps over half, of these nine Middle and Upper Silurian species are synonyms. 
