302 



DR. H. LYSTER JAMESON ON 



margin, and arises in the normal manner in a marginal groove 

 (Ceylon Report, Part II. Anatomy of Pearl-Oyster, plate viii, 



fig;2). 



Text-fig. 35. 



3Iargaritifera vulgaris, Pei'sian Gulf. Section through the lip of the shell, after 

 decalcification, showino: the successive lappet-like processes of tlie prismatic 

 substance (?., I.', I.", I.'"), pr., prismatic substance ; Str., stratification of same. 

 A, A', A", A'", points marking the successive retreats of the secreting margin, 

 which takes place vi^hen new lappets are to be formed. At these points the 

 " periostracum " of the new lappet is continuous with, and indistinguishable 

 from, the inner conchyolin-la_yer of the prismatic substance of the last-formed 

 lappet. Preparation X (X 35), see also PI. XXXIV. fig. 6. 



(ii.) The Prismatic Substance. 



Reduced to its simplest terms the prismatic layer of the 

 Mother-of-Pearl shell consists of prisms of calcium carbonate 

 perpendicular to the surface of the shell, bounded externally and 

 internally by membranes of conchyolin, which are connected by 

 vertical membranes forming the septa between the prisms 

 (text-figs. 35 and 36 ; see also PL XXXIV. figs. 6, 6 a ; PI. XL. 

 fig. 29 a). Where interruptions in the continuity of the growth 

 of this layer have occurred, the layer of prisms may be divided 

 by one or more horizontal walls of conchyolin (text-figs. 35, str., 

 and 38, str.), which break up the prisms into segments, or which 

 divide the layer into two or more series of prisms. The individual 

 ends of the prisms in one layer do not of necessity coincide with 

 those of the prisms in the next layer, though they frequently do 

 coincide. The septa between the prisms may also show annular 

 thickenings, corresponding to zones of constriction around the 

 prisms (PL XXXIV. fig. 6 a, ann.). Romer has shown (32, p. 35) 

 that the prisms of the pearl-shell (like those of Pinna, described 

 by Biedermann (1), p. 9) behave between crossed nicols in the 

 same manner as single crystals. The prisms difier enormously in 

 size, according to the age of the oyster and the conditions under 

 which they are secreted. 



There is a sharp line of demarcation between the prismatic 

 and the nacreous layers in Margaritifera, the innermost layer of 

 the conchyolin of the former being connected to the outermost 

 layer of that of the latter by a series of fine connectives of 

 conchyolin (PL XXXIV. fig. 6 a, con.), forming a curious alveolar 

 layer. 



