210 MICROPOHELLID^. 



our own seas, the vibraculoid mandible of the avicularium 

 is exceedingly slender and delicate, and the ooecium umbo- 

 nate. In the Australian form, the base of the oral spines 

 is black, a peculiarity which I have not noted in British 

 specimens. Another marked variety has the surface 

 thickly covered with well-defined granules, the avicu- 

 larium very large, a prominent mucro below the mouth, 

 concealing the median pore, and a prettily areolated 

 ovicell. In an Algerian specimen, which incrusts a stem, 

 free expansions are given off, composed of two layers of 

 cells placed back to back, and resembling a foliaceous 

 Eschai-a of authors. 



The form from the English Coralline Crag and the 

 Italian Pliocene beds corresponds completely with the 

 recent deep-water variety, distinguished by the thick gra- 

 nular wall of the cells. 



But the most interesting variation to which this sj)ecies 

 is liable, is that which takes place in the avicularium. 

 In some cases, and especially in the thick-walled granular 

 form, this organ exhibits the usual structure; in others, 

 and chiefly in the more delicate varieties, the mandible is 

 lengthened and attenuated, so as to take on a distinctly 

 vibraculoid appearance. No alteration, however, takes 

 place in the fixed, beak-like portion of the structure ; and 

 the mandible is therefore incapable of any true vibra- 

 cular function. The two conditions of the avicularium are 

 sometimes met with in one and the same colony. We have 

 here an indication of the way in which the avicularium 

 may pass by gradual modification into the vibraculum. 



The pore beneath the orifice is either den- 

 tate or filled in by a fine calcareous network ^^" 

 (a beautiful structure), which must effectually (^% 

 guard the entrance to the interior of the cell 



^ Pore (Califor- 



(woodcut, fig. 9) . nian var.)- 



