Genera of the ^N'oeth American Palaeozoic Bryozoa. 435 



bases of vibracula or of the smaller avicularia. In support of 

 his theory Prof. Nicholson gives the following figures : 



b 



M'! W%^ 



Fig. 15. a, A transverse section of a recent species of RKTEroRA, taken parallel to the noncellu 

 liferous surface of the frond, enlarged ; showing the thickened tubes p, to which the avicularia were 

 attached ; h, A section of Coscinium {Coscinotrypa) cribriformis, from the Devonian rocks of Canada, 

 showing similar thickened tubes; c, A section of Rhombopora {Ceriopora) Hamiltonensis, showing 

 similar tubes (after Nicfiolson). 



The following discussion is from Hiscks' " British Marine Poly- 

 zoa, Yol. I. Introduction, pp. Ixiv-lxxxiii, inc., 1880." 



" MoDIFiOATIONS OF THE ZoGEOIAL TyPE. 



" The structural type of which the zooecium is the most famil- 

 iar representative, exhibits a number of modifications amongst 

 the marine Poltz >a. Of these the most remarkable are the 

 avicularium and vibraculum. These curious appendages are 

 confined to a single saborder, the Cheilostomata, within the 

 limits of which they occur in great abundance and variety. The 

 vibraculum (probably a derivative from the avicularium) is 

 rarely met with as compared with the latter, which is present in 

 a large proportion of the Cheilostomatous genera. The avicu- 

 larium is best known in its most highly specialized form as it 

 occurs in the genera B^gula and Bicellaeia. 



" This is the true ' bird's head,' an articulated appendage at- 

 tached to the zooecium, with a formidable hooked beak and a 

 mandible worked by powerful muscles, perpetually snapping its 

 jaws with a monotonous energy, and swaying to and fro with a 



