328 DR. H. LYSTER JAMESON ON 



PI. XXXYIII. fig. 19 sliows a group of such pearls, scattered 

 among which are hypostracum-pearls {h//.]).). The nucleus of 

 the eccentric pearl at nit.' is clearly composed, like that of the 

 incipient pearl shown on PI. XXXIX. fig. 22, 7iac., of hypo- 

 stracum, and resembles the hypostracum-pearl shown at fig. 21, 

 which has a small quantity of granular contents. One of the 

 columnar nuclei is shown enlarged at fig. 20. This is the nucleus 

 of the large pearl shown in the upper part of fig. 19 

 (PI. XXXVIII,). Centrally there is a small cavity, in which a 

 granular mass of doubtful origin is noticeable (gr.). This cavity 

 is surrounded by an irregular zone of substance which seems to 

 be the same as the amorphous repair-substance of the shell 

 {cf. PI. XL. fig. 29). Externally to this there is columnar 

 substance, forming a sort of sphserocrystal-like mass, the fibro- 

 crystalline calcium carbonate being deposited in a conchyolin-like 

 basis, which, on decalcification of the pearl, remains as a 

 framework. This substance is seen cut tangentially at one end 

 of the pseudo-nucleus. Outside this is the normal nacre of the 

 pearl. In one or two cases the amount of granular matter in the 

 centre of the pearl was considerable ; thus PI. XL. fig. 24, which 

 is taken from the same series as the pi-eceding figures, shows the 

 central portion of a pearl which has a dumbbell-shaped double 

 nucleus, the larger half being about "17 mm, in diameter, and 

 each half containing an opaque brownish mass of dead animal 

 matter, perhaps of cellular origin, but quite unrecognisable. The 

 contents of the two cavities w^ere continuous at one point. There 

 is nothing to suggest that either of these bodies represented a 

 dead Cestode ; I could find no denser portion, such as would 

 naturally occvu' at the point representing the myzorhynchus, nor 

 was there anything that could be safely identified as the cuticle 

 of the parasite. If the contents suggested anything, it was rather 

 the remains of a large Protozoan parasite, containing spore-like 

 bodies in a plasma (like the Sporozoan spores which Dubois claims 

 to have identified in the nucleus of a pearl from M. vulgaris fi'om 

 the Mediterranean, 7, p. 311, and 8, p. 104) ; or, perhaps, a mass 

 of mucus containing a few cells. Each of these centres was 

 surrounded immediately by the amorphous substance, this being 

 followed by, and continuous wath, a radially calcified layer, the 

 bases of the calcareous j)risms being evidently embedded in the 

 amorphous substance, which extended up between them to form 

 the organic framework of the columnar substance. This condition 

 is comparable to that seen at tr. on PI. XLI. fig. 30, which shows 

 the amorphous substance secreted at the junction of two pearls 

 in a compound pearl, passing over into columnar repair- 

 substance. 



Externally to the columnar layer of the pseudo-nucleus the 

 normal nacreous layers of the pearl are formed. 



The other extreme is shown on PI. XL. fig. 25, a hypostracum- 

 pearl "08 mm. in diameter, the central cavity of which has no 

 contents vvhatever. 



