162 DR. H. LYSTER JAMESON ON THE [Mar. 4, 



hippopus, in which species the shell is thick and massive, are 

 often as large as a cherry. Among the true Pearl-Oysters the 

 large thick-shelled species, Margaritifera maxima Jameson and 

 M. margaritifera Linn., produce tlie largest pearls, those yielded 

 by the smaller M. vulgaris (Schumacher) on the Ceylon fisheries 

 and elsewhere being usually small, and prized i-ather for their 

 colovir and lustre than for their dimensions. 



And it is interesting to note that M. vulgaris in the Persian 

 Gulf, where it attains larger dimensions and yields a more massive 

 shell than in Ceylon, gives on an average larger pearls than the 

 Gulf of Manaar pearl-oyster. 



The general experience of everybody acquainted with pearl- 

 fisheries is that the largest pearls are found in the oldest and 

 thickest shells, which proves how intimately the growth of pearl 

 and shell are associated. It is natural that such an association 

 should exist, since, as is obvious from the results recorded in this 

 paper, the mechanisms of both processes are the same. 



Origin of Pearls in other Forms. 



Two questions will naturally occur : — Are we warranted in 

 assuming that the mechanism of pearl- formation is the same in 

 other molluscs ? and, Is it generally caused by Trematode larvae ? 



In answer to the fii-st question, I may say that in those cases 

 where I have been able to examine pearls in siiio, in Margaritifera 

 margaritifera Linn., M. vulgaris (Schumacher), M. maxima 

 Jameson, Rippopus hippopus L., and Pinna nigrina Lam., this sac 

 is universally present. It has been noticed by von Hessling in 

 Margaritana margaritifera, and by Diguet in Margaritifera 

 margaritifera L., from California. But, apart from this evidence, 

 it is safe to say that without such an epithelial sac to shed the 

 cuticnlar conchyolin, the nacreous layei-s of the pearl could not be 

 laid down at all. 



To what extent other causes besides Ti'ematode larvfe may be 

 capable of inducing such sacs to develop, has yet to be ascertained. 

 Trematodes have been unquestionably associated with peail- 

 formation in Anodonta, Mai^garitana margaritifera, Mytilus 

 edulis, and Margaritifera (?) mazatlanica (see Introduction). 



Besides these records, I have detected the remains of Trematodes 

 in decalcified or sectioned pearls from the following species : — 

 Margaritifera margaritifera Linn., M. maxima Jameson, M. vid- 

 garis Schumacher, Pinna nigrina Lam., P. euglypta Hanley, 

 P. virgata Menke, Rippopus hippopus Linn., Tridacna gigas 

 Lam., and Mytihis magellanicus. 



I examined pearls from several other molluscs, but had not 

 sufiicient material to ascertain satisfactorily. In M. vtdgaris 

 Schumacher, besides the Trematode, there seems to be a second 

 organism, possibly a Gregarine, concerned in pearl-formation. 



The periostracum pearls in the mantle-margin of Modiola are 

 also associated with parasitic protozoa. 



