D V. .. . , BEYOZOA. 147 



Pachydlctya ] , 



the Anticosti group; crassa (Hall), bifurcata{Yan Cleve), emaciata (Foerste), fardus 

 (Foerste), and rudis (Foerste), from the Clinton, and scitula (Hall) from the Niagara.* 



In placing Pachydidya under the Bhinididyonidce 1 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 

 Hlinois, 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 Rhinididya 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 Pachydidya and the 

 new. genus Trigonodidya, 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 Bhinididyonidce. The difficulties are encountered when we 

 attempt to draw sharp lines between certain species of Pachydidya on the one side, 

 and Phyllodidya and Eurydidya 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 Bhinididyonidce 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 Pachydidya and Trigonodidya, 

 and in a lesser degree also Phyllodidya, differ from the more typical members of the 

 family : Bhinididya, Dicranopora, Goniotrypa, and Eurydidya. In all of the latter 

 the primitive or prostrate portion of the zooecial 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 Eurydidya. 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 zooecial cavity to cavity, there being no sharply defined ring-like wall around 

 the latter. 



In Pachydidya, Trigonodidya and Phyllodidya, 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. 



»I am convinced that several, perhaps over halt, ot these nine Middle and Upper Silurian species are synonyms. 



