334 DR. H. LYSTER JAMESON ON 



Turning now from the " nuclei " of these peai-ls to the " pseudo- 

 nuclei " which surround them, these do not appear to present any 

 characters which cannot be expressed in tei'ms of the several 

 pathological varieties of shell-substance described above. Thus 

 PI. XLIII. fig. 44 shows the nucleus (composed of diatoms, etc.) 

 surrounded by typical amorphous and simply columnar repair- 

 substance, followed by ordinary nacre. This appears to be 

 equally the case in PI. XLVI. fig. 55, where the nucleus is a 

 sand -grain. 



PL XLI. fig. 35 and PI. XLV. figs. 50 & 50 a, which had 

 no foreign nucleus, PI. XLII. fig. 36, PI. 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 XLY. 

 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 (Pis. XLII. & XLIII.) show it 

 highly stratified, and presenting every gradation from amorphous 

 to prismatic substance. This preparation is a " brown peai-l," 

 lather over 3 mm. in diameter, composed of the prismatic shell- 

 substance, and probably produced in the mantle-margin. It 

 consists of an outer zone, cieai^ly 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. 4:2, pr. Inside this noi-mal 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, co^.). 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 (tigs. 40, 41, am.). In 

 places these bodies are i-anged in radial rows, and the amorphous 

 substance nearly reaches a stage which could be called columnar 

 substance. 



Just ai'ound the centre (fig. 40, col.) typical columnar substance 

 occurs, passing over internally into amoi'phous substance. 

 Owing to the impenetrability of the amorphous substance, the 

 calcium carbonate has not heen 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 XLIII, fi'om which figures 50 & 50 a on PL XLV. 

 and fig. 35 on PL XLI. are drawn, showed an oval central body, 

 about "6 mm. long, forming a "pseudo-nucleus."' On superficial 

 examination this nucleus is highly suggestive of a Ti'ematode 

 (fig. 50). But sections showed it to consist of a small sph?ero- 

 crystal-like body (fig. 35) about "08 mm. in diameter, surrounded 

 by granular repair-nacre. 



