a General History of the Marine Polyzoa. 89 



(beak) made up of two opposed calcareous plates, which slope 

 up to a central groove, narrow and linear above, expanded 

 towards the base ; the mandible elongate, triangular below, 

 running out into a slender pointed process. cerium (?). 



Loc. Australia, incrusting weed. 



This species was long ago described by Lamouroux. I 

 have figured it in orde*r to show the remarkable avicularium, 

 which has not hitherto been specially noticed. It illustrates 

 very strikingly the homological nature of this appendage : the 

 area on which it is placed exactly resembles that of the ordi- 

 nary zooecia, except that it is a shade smaller ; the avicularium 

 itself occupies the position of the orifice, of which it is plainly 

 a modification. The basal subtriangular portion of the man- 

 dible is the equivalent of the opercular valve ; the setiform 

 process into which it is prolonged above is the superadded 

 element. The calcareous plates which enclose the groove 

 into which the mandible falls are a modification of the cal- 

 careous border of the zocecial orifice, and possibly of the 

 lateral spines. 



The very definite character of the operculum in this species 

 and the thickened border by which it is enclosed are points 

 of much significance. The orifice is very much that of the 

 Escharine (Smitt) or old Lepralian group. We have in this 

 form and others like it an intermediate stage between the 

 simple Membraniporidan structure and that of the Micro- 

 poridaBj in which a similar orifice is associated with the com- 

 plete calcification of the front wall. It may be a question 

 whether the structural peculiarities of the present species 

 should not be made the basis of a distinct generic group *« 



It may be noted that on the outskirts of the colony abnor- 

 mal cells are sometimes met with, in which the space usually 

 filled by the orifice and the two spines is occupied by a broad, 

 continuous, calcareous plate, stretching from wall to wall 

 across the entire width of the zooecium. 



Membranipora transversa. (PI. XI. fig. 9.) 



Zooecia elongate, rectangular, disposed in regular transverse 

 (and longitudinal) series ; aperture occupying more than half 

 the front of the cell, with a wholly membranous covering ; 

 margin thin, smooth ; orifice arched above, lower margin 

 slightly curved outwards, surrounded by a definite border, 

 on each side a very stout blunt spine, white and lineatcd 



* In the normal Membraniporce the orifice is a simple semicircular 

 opening in the front wall, and the operculum only differs from the uiciu- 

 branoufc covering in being of slightly firmer substance ; it docs not work 

 on a distinct hiuge. 



