PORELLA CONCINNA. 325 



surface is eveu and uniform. But tlie avicularia exhibit 

 the most remarkable variations. They are sometimes 

 placed rather deeply within the mouth, and sometimes on 

 the outer edge of the inferior margin, crowning the umbo 

 immediately below it. Occasionally they are developed 

 in large numbers, and distributed irregularly over the 

 zoarium, each avicularium supported on a small mound- 

 like prominence (Plate XLVI. fig. 12). At times they 

 are modified in shape, as in Schizoporella auriculata, S. 

 tirmata, and probably in many other species. The small 

 circular avicularia are replaced, at least in a considerable 

 number of the cells, by oval or spatulate avicularia of 

 much larger size, which often cover a great part of the 

 front surface (Plate XLVI. figs. 5 & 10). 



Occasionally a large avicularium is present on a cell 

 which bears one of the ordinarv form and in the normal 

 position. I have seen one mounted on a mamillary emi- 

 nence which occupied the whole of the centre of the cell, 

 with its mandible directed upwards, and almost in contact 

 with the normal avicularium on the lower lip (Plate XLVI. 

 fig. 10). Generally, however, the larger form replaces 

 the latter, of which it is evidently a modification. These 

 remarkable changes illustrate still further the instability 

 of this structural element, to which reference has so often 

 been made. 



In the variety (a) described by Dr. Dawson as a species, 

 in his valuable paper on the Invertebrata of the Gulf of 

 St. Lawrence, the zooecia have a somewhat sinuous out- 

 line, which changes very materially the general appear- 

 ance, but has no significance as a specific character. The 

 wall of the cell is carried up on each side of the mouth 

 into a stout digitiform process — a condition which is met 

 with in other varieties. 



Very commonly in the fertile cells these portions of the 



