322 BH. n. LYSfER JAMESON ON 



depositions or calcosphernles * arise in the tissues, close to the 

 attachments of the muscles to the shell. Ectoderm-cells may 

 " mii^Tate to the source of irritation, and thus he responsible for 

 the deposition of a pearl." No explanation of the origin of these 

 calcosphernles is given, hut Mr. Southwell thinks it is "almost 

 certain that they are depositions fi'om the blood," and refers to 

 them elsewhere as " of excretory origin " (42). 



I have been led by my observations to take a quite different 

 view of these " calcosphernles " f, and as their oi-igin is so closely 

 related to that of Muscle-Pearls, 1 cannot do better tlian begin 

 the present section of my paper with an account of theii- struc- 

 ture and origin. 



According to my view, Prof. Herdman's " calcosphernles " are 

 not free concretions at all, but are minute pearls, composed of 

 hypostracum ; and I propose, therefore, to call them " hypostra- 

 cum mnscle-peai'ls," to separate them from " nacreous muscle- 

 pearls." As stated by Herdman, these bodies occur close under 

 the epidermis (unless secondarily displaced, e. g. b}^ the addition 

 of new ones), and I usually find them in the region y.'hei'e the 

 muscle-attachment epithelium passes over into the ordinary shell- 

 secreting epidermis of the mantle. A gi'oup of these hypo- 

 stracum-pearls is shown on PI. XXXIX. fig. 22, which represents 

 a portion of the mantle-musculature of one of the uidabelled 

 specimens in the British Museum, examined entire in oil of 

 cloves. The same pearls, decalcified, are seen in fig. 23. These 

 little bodies measured from 0*02 to 0*5 mm. in diameter. In 

 PI. XXXVIII. fig. 19 similar bodies, hy.p., are seen in a section 

 along with ordinary nacreous muscle -pearls; while single indi- 

 viduals are shown "in PI. XXXIX. figs. 21 & 21 a and>l. XL. 

 fig. 25. Sections ground from these bodies, or cut from the organic 

 residues left when they are decalcified, show them to be composed 

 of the same substance as the hypostracum of the shell. They 

 consist of calcium carbonate, in fine fibrocrystalline form, showing 

 radial and also concentric markings, with a small central cavity 

 (PI. XXXIX. fig. 21 a). Decalcified they also resemble hypo- 

 stracum in all details of structure and reaction to stains 

 (fig. 21). Their organic basis stains more blue with ha?matoxylin 

 than the organic pai'ts of the other shell- substances, and takes up 

 carmine more deeply. Their alveolar structure is also miach finer 

 than that usually found in the columnar varieties of repair- 

 substance, so fine, in fact, that in surface-sections the reticular 

 structure seems almost like that of the protoplasm itself. As has 

 been observed in the hypostracum of the shell, this substance 

 sometimes passes over into nacreous conchyolin laterally. The 



* This word is presumably intended to convey the same idea as the word 

 " concretion " adopted by me (25) in 1902, i. e. a sph;i>rocrysta.l-like body arising in 

 the tissues otherwise than by epidermal secretion ; and therefore analogous to 

 cholesterin calculi, etc. {cf. Hartiug's " Calcosphierites," 12). 



t I ilnd that Rubbel (34 a), working on the freshwater Pearl-Mussel, 

 7lf«»'(7rt?v'i'ff?((r, has arrived independently at the same view of the nature of these 

 bodies as that here propounded. 



