282 Rev. T. Ilincks's Contribatioas towards a 



bounded bj thick and conspicuous raised lines ; suvfaca 

 flattisli (sutures very shallow), punctured; orifice (primary) 

 arched above, constricted by a prominent projection on each 

 side, a short distance above the lower margin, which is slightly 

 curved outwards ; peristome elevated, especially at the back 

 and sides (forming a raised rim), in front rather broad and 

 somewliat flattened, unarmed ; operculum smooth and polished, 

 narrow (not so wide as the oritice), the space between it and 

 the margin tilled in by a horny plate, sliglitly hollowed out 

 at the sides a little above the inferior margiu. Ooe:iiiin large, 

 suborbicular (rather broader than high), very moderately con- 

 vex, punctured, a raised line round the base, witliin which 

 there is a row of larger pores ; peristome carried across the 

 front of the ovicell, often rising into a projection oa each side of 

 the orifice. Immediately under the lower margin (in the ooecial 

 cells) a large aviculariam^ broadly spatulate, short, contracted 

 at the base, the mandibular portion much expanded and 

 rounded anteriorly, mandible directed downwards ; the space 

 behind the mandible marked off by a very prominent denti- 

 cular process on each side. 



Log. Port Phillip Heads (/. B. Wilson). 



A. curious peculiarity in this species is the want of corre- 

 spondence in size between the orifice and the operculum. 

 The latter is very narrow, and there is a space between it and 

 the margin which is filled in by a delicate chitinons expan- 

 sion. A distinct depression or furrow runs across the oper- 

 culum in the line of the lateral denticles, and marks the 

 hinue on which the movable valve workr Tiie denticles are 

 unusually large and prominent. 



Avicularia seem to be altogether waivii/ig on the cells 

 which are not furnished with ooecia. On those which bear 

 ooecia they are present and exhibit a very distinctive form and 

 structure. They originate immediately under the lower 

 margin of the orifice, and occupy a large portion of the area 

 of the cell. The mandible is short and unusually broad and 

 suborbicular in shape. 



In most other cases the chamber or basal portion of the 

 appendage is separated from the mandible by a partition, 

 on which the latter works ; but here two large denticles con- 

 stitute the supports on which the valve moves, correspond- 

 ing with those which occur in the orifice of the cell. The 

 avicularium without its mandible resembles very markedly the 

 orifice without its operculum reversed. 



