LIFE-niSTOET Or MAEGAEITIFERA PANASESiS. 413 



the latter part of tlie existence of this bivalve. On the other 

 hand, the formation of new byssi is possible, and, if occasion 

 demands it, always occurs in shells of almost any age. 



Degeneration consequent on a more sessile existence is ob- 

 servable, in this species, in the atrophy of the modified foot 

 during the latter parb of the bivalve's existence. The pi'esence 

 of rudimentary hiuge-teeth in this species is also worthy of 

 notice. 



I have been unable to observe any indications of independent 

 movement in the spat, or young shell, of the New Gruiuea variety 

 of the black-edged Molher-of-Pearl Oyster {Margaritifera mar- 

 garitifera). As in M. panasescE, its byssus is permanent and 

 persists throughout life, being instrumental in keeping tlie 

 oyster in au upright position. In the event of accident a new 

 byssus is formed (almost at any age, as in If. panasesoe), pro- 

 vided that suitable anchoring material exists within reach. This 

 species possesses no trace of hinge-teeth. 



In the white- or golden-lipped Mother-of-Pearl Oyster (^Mar- 

 garitifera maoaima), of commercial importance, no movement is 

 reported as having been observed in the young shell or spat. 

 A byssus is formed, but it is not a permanent one, and does not 

 persist beyond the ninth month or so. After that age, and for 

 its stability, the shell relies on its weight and lies, unattached 

 in any way, on the bottom. This species also possesses no traces 

 of hinge-teeth. 



"With these three types it is possible to form an interesting 

 series illustrative of the modifications which have taken place 

 Avithin the subgenus Margaritifera. M. panasesce, with its 

 rudimentary hinge-teeth, permanent and renewable byssus, whose 

 formation is preceded by locomotion by means of a modified foot, 

 may be taken as a type possessing the most primitive character- 

 istics. M. onargaritifera, showing absence of hinge-teeth and 

 foot, but presence of permanent and renewable byssus, may be 

 taken as a more advanced type. M. maxima, with absence of 

 hinge-teeth and foot and with only a temporary byssus, may be 

 taken as a type representing the highest development among the 

 Mother-of-Pearl Oysters. Further iuvestigations in connection 

 with intermediate forms would no doubt make the series more 

 complete. 



Perth, W.A., Aug. 12, 1905. 



