304 DR. H. LYSTER JAMESON ON 



Molluscan shell results, the axes of its constituent elements being 

 approximately parallel. 



(iii.) The Nacre. 



This substance, which forms the bulk of the shell, and gives 

 the shells of the genus Margaritifera their commercial value as 

 Mother-of-Pearl, and the pearls their beauty, is stratified, and 

 in it the calcium carbonate is divided into extremely minute 

 bodies in the organic network. It is secreted by the outer surface 

 of the mantle and body-wall. 



I will not attempt here to review the many writings on the 

 structure of this layer. I can at present add little to the recent 

 work of Romer (32), who has studied its structure and that of its 

 decalcified conchyolin framework very thoroughly. The organic 

 basis which gives it its form, and which retains its iridescence 

 after the calcareous salts have been extracted, consists of a series 

 of parallel lamell?e, of extreme fineness, united to one another 

 at intervals by radial connections, so as to form a series of minute 

 flat or lenticular chambers, separated by organic walls of extreme 

 delicacy. The calcium carbonate appears to be enclosed in 

 these chambers in the form of little polygonal plates or lozenges. 

 This structure is difficult to observe, owing to the distorting 

 effect of the decalcification process, which, owing to the evolution 

 of gas-bubbles, tears some lamellae apart and forces others 

 tightly together. It becomes much more obvious in some of 

 the abnormal and pathological varieties of nacre described as 

 "repair-substance" below, notably in "granular repair-nacre." 



I believe that the lustre (not the iridescence) of mother-of-pearl, 

 and of pearls, is in great measure due to the fact that each of 

 these tiny plates is a minute, biconvex lens; and that the 

 extraordinary and indescribable character of the light reflected 

 fi'om the surface of a fine pearl is in part the cumulative expression 

 of the action of these myriads of little lenses upon the light 

 reflected from the surfaces of calcium carbonate and of conchyolin 

 which underlie them. 



(iv.) Hyijostracum^ or Muscle- Attachment Substance. 



This curious substance has not secured all the attention it 

 deserves. It has been described by several writers under the 

 names Hypostracum (Thiele, 46), Stabchenschicht (F. Miiller, 

 29), durchsichtige Substanz (Ehrenbaum, 9 ; Tullberg, 47). 



I retain the name hypostracum, as emphasizing the distinct 

 origin and characters of this layer, and as shorter and more 

 convenient than " durchsichtige Substanz." 



This hypostracum is a fine columnar layer forming the surfaces 

 where the muscles are inserted into the shell (PI. XXXIY. figs. 5 

 & 7, Ay. ; text-figs. 36 & 37). 



It is more ti-ansparent than the nacre — indeed, the iridescence 

 and lustre of the muscle-scar is due to the nacre lying below and 



