THE CEYLON PEARL-OYSTER. 303 



The prismatic sul>stance is secreted by tliat part of the epidermis 

 apposed to the inner sui'face of the shell which is nearest to the 

 margin of the mantle, and it is clear from the rapidity with 

 which tlie columnar lip and its lappet-like processes are re- 

 genei-ated when injured and are added to in growing young shells 

 that the characters of this layer are associated with relatively 

 rapid secretion. This is significant, in view of the resemblance 

 of this layer to some of the rapidly secreted repair-substances 

 which replace the nacre under certain abnormal conditions 

 (see below). 



Romer's work (p. 18) gives an interpretation of the nature of 

 the prisms, which 1 have found most useful in helping me to 

 interpret my own observations on pearl -formation. According to 

 this hypothesis, which was suggested to Rcimer by my illustrious 

 teacher. Prof. Biitschli, each prism is an incomplete sjjhserocrystal, 

 the growth of which has been arrested in all directions but one, 

 viz. the direction from which the new shell-substance is secreted. 

 Riimer says (p. 18) : — 



"Dass diese Spharokristalle der einzelnen Prismen so 

 unvollstiindig ausgebildet sind, riihrt daher, dass gleichzeitig 

 und dicht nebeneinander die Anfjinge der einzelnen Prismen 

 oder .Spiirokristalle gebildet wurden, die bald seitlich 

 aufeinander stiessen und sich so gegenseitig in der weiteren 

 Ausbildung hemmten ; nur an ihren inneren Endeu 

 vermochten sie einseitig weiter zu wachsen " *. 



If I may be allowed to state the proposition in slightly 

 difierent terms, the prismatic shell-substance (and, indeed, if my 

 interpretation of the variations of the nacre, normal and patho- 

 logical, are correct, the whole of the shell- substance) agrees with 

 a .sphserocrystal in that it is composed of crystalline or crystallised 

 substance which can only grow by the apposition of fresh layers 

 deposited on a single surface, owing to the matter in solution 

 only having access to one surface of the crystalline mass. When 

 this surface is the outer surface of a sphere, a body with more or 

 less of the characters of a sphperocrj'stal results (e.g. Harting's 

 bodies and Pearls) ; where it is approximately a plane surface, as 

 in the growth of the MoUuscan shell, a structure such as the 



* Biitschli in 1908 (6, p. 26) explained bis definition of "sphaerocrystals," more 

 especially with reference to tlie crystal-like prisms of Pinna (and by analogy 

 of 3Iaryaritifera), as follows : — 



" Wie aus den Darlegungen in meinem Werk von 1898 bervorgeht, verstehe 

 ich untereinen solcbennicbt ein Aggregat zentrisch angeordneter Kristallnadeln 

 oder Einzelkri^stalle, was zwar die iibliche Anschauung ist, sondevn ein 

 einbeitliches Kristallgebilde, in welchem die besondereu feinsten Struktur- 

 verhaltnisse, die aucb den gewohnlichen Kristallen ihre charakteristichen 

 Eigenschaften verleiben, nicht entsprecbend einer Axe, sondern um ein Zentrum 

 radiar orientiert sind. Wenn daher der Radius eines solcben Spbiirokristalls 

 sehr gross wird, und man ein radiales Stiick desselben, weit entfernt von dem 

 Zentrum, herausschneidet — und so verbalten sich etwa die Pinnaprismen — so 

 muss dieses Stiick sich natiirlieh wie ein gewiJbnlicber Kristal verbalten ; 

 obgleich die von mir gegebene Zuriickfiihrung auf einen Spharokristall niit 

 grossem Radius ganz zutreffend ist." 



