Genera of the N'orth American PxVLaeozoic Bryozoa. 433 



sar}^ that some species need a double means of defense, while 

 other species equally fragile are destitute of any. 



The compound forms of vibracula may develop into an aviou- 

 laria, according to Hinck8 who, in The Annals and Magazine of 

 Natural History, 1^81, cites the case of the nearly ubiquitous 

 species Microporella ciliata. Fig. 13, a, 5, c, d^ shows the transi- 

 tion from avicularia to vibracula; a shows a cell with a sessile 

 avicularium of the ordinary type ; in b the mandible is prolonged 

 into a slender spine, in other respects resembling a; in c the man- 

 dible has entirely lost its lid-like character, and is prolonged into 

 a long, slender process, tapering to a point, and analogous to a 

 vibraculum. The beak has also undergone a slight modification? 

 recalling the vibracular cell which supports the movable seta. 

 In d another modification is seen; the mandible is prolonged into 

 a narrow spine, and on each side of it there is a membranous 

 expansion, forming a flapper. 



Fir.. 13. Microporella ciliata ; a, b, c, d, showing modificatious of a sessile avicularium (after Hincks). 



In the fossil state the obverse or noncelluliferous face of Fenes- 

 telloid forms is apparently a solid calcareous layer ; but in a thin 

 transparent section it is seen to be penetrated by numerous 

 minute tubuli at right angles to the surface. In a decorticated 

 specimen of the same family, the longitudinal structure is appar- 

 ently composed of numerous cylindrical fibers. In a cross-section 

 these have the appearance of tubuli. Many of the cells, both in 

 55 



