334 DR. H. LYSTER JAMESON ON 
Turning now from the “ nuclei” of these pearls to the “ pseudo- 
nuclei” which surround them, these do not appear to present any 
characters which cannot be expressed in terms of the several 
pathological varieties of shell-substance described above. Thus 
Pl. XLII. fig. 44 shows the nucleus (composed of diatoms, etc.) 
surrounded by typical amorphous and simply columnar repair- 
substanee, followed by ordinary nacre. This appears to be 
equally the case in Pl. XLVI. fig. 55, where the nucleus is a 
sand-grain. 
Pl. XLI. fig. 35 and Pl. XLV. figs. 50 & 50a, which had 
no foreign nucleus, Pl. XLII. fig. 36, Pl. XLV. fig. 52 with 
granular contents in a central cavity, and Pl. XLII. figs. 37-39, 
in which example the nucleus was a sand-grain, and Pl. XLV. 
figs. 51 & 53 and Pl. XLVI. fig. 56, show these pseudo-nuclei to 
be composed of varieties of the columnar substance, Fig. 57 
(Pl. XLVI.) and figs. 40-42 (Pls. XLII. & XLII.) show it 
highly stratified, and presenting every gradation from amorphous 
to prismatic substance. This preparation is a ‘ brown pearl,” 
yather over 3 mm. in diameter, composed of the prismatic shell- 
substance, and probably produced in the mantle-margin. It 
consists of an outer zone, clearly identical in characters with the 
prismatic layer of the shell, and showing three distinct series of 
prisms. The bases of the innermost layer of prisms are shown in 
fig. 42, pr. Inside this normal prismatic substance is the “ pseudo- 
nucleus,” which consists externally of a layer resembling in every 
respect, except the diameters of its constituent elements, the 
prismatic layers (figs. 41, 42, col.). Internally this passes over, 
through a substance resembling irregular columnar repair-substance 
(fig. 41, tr.), into amorphous substance containing columns or 
scattered depositions of carbonate of lime (figs. 40, 41,am.). In 
places these bodies are ranged in radial rows, and the amorphous 
substance nearly reaches a stage which could be called columnar 
substance. 
Just around the centre (fig. 40, col.) typical columnar substance 
occurs, passing over internally into amorphous — substance. 
Owing to the impenetrability of the amorphous substance, the 
calcium carbonate has not been dissolved in the deeper layers of 
the pseudo-nucleus, and it is seen in the nucleus itself to occur in 
the form of perfect rhombohedra, a form I have never seen it 
take on in the alveoli of the normal or abnormal shell-substance. 
The resemblance of this pearl to the brown Scotch river-pearls 
(e. g. that figured by me in ‘ Nature’ for Jan, 22nd, 1903, p. 281 
{26}]) is striking. 
Preparation XLII, from which figures 50 & 50a on Pl. XLV. 
and fig. 835 on Pl. XLI. are drawn, showed an oval central body, 
about *6 mm. long, forming a ‘“ pseudo-nuecleus.” On superficial 
examination this nucleus is highly suggestive of a Trematode 
(fig. 50). But sections showed it to consist of a small sphero- 
erystal-like body (fig. 35) about ‘08 mm. in diameter, surrounded 
by granular repair-nacre. 
