NEWTOJT : FOSSIL PB^ARL-GROWTHS. 129 



beds of the so-called Cambridge Greensand. The sides of these casts 

 were found to be frequently covered with rounded pittings, the origin 

 of which was fully acknowledged by the author to be due to the 

 former presence of pearls on the original interiors of the valves. 



Another internal cast of an Inoceramus {Inoceramus sagensis^ var. 

 quadrans) with rounded cavities, from the Cretaceous rocks of 

 New Jersej', was iigured by Professor E.. P. Whitfield* in 1885. 

 Somewhat similar pittings were figured by Professor Yokoyama'* as 

 being present on an Inoceramus cast from the Upper Cretaceous rocks 

 of Japan, and mention may also be made of isolated pearls occurring 

 in the marine Post-Pliocene deposits of the Clyde Basin at Garvel 

 Park, which have been referred to by Mr. D. Robertson.^ 



Asa final statement under this heading, reference may be made 

 to numerous minute sphei'ical bodies varying in diameter from 

 ToVo to a^o" of an inch, which were detected in some examples of 

 Chalk from Cherry Hinton near Cambridge, Swaffham in Norfolk, 

 and other localities, by C. B. Rose* during 1859, who mentioned 

 their occurrence in connection with some observations on " Geological 

 Pearls." Mr. Rose was of opinion that if these minute sphei'es 

 were to be recognized as pearls, then thej^ might be considered a^ 

 'seed-pearls.' Later investigations have, however, proved that 

 such structures are not pearls, but represent the disunited cells of 

 Globigerinse and other Foraminiferal remains, a fact clearly demon- 

 strated by Messrs. W. Hill and A. J. Jukes-Browne,* whose microscopical 

 researches have included the examination of specimens of chalk 

 obtained from the same localities as those referred to \)j Mr. Rose.® 



II. Occurrence and Formation oe the Pearl. 



According to S. P. Woodward's "Manual of the Mollusca," pearls 

 are found in quite a number of shells, the chief of which are the 

 Oriental pearl-oyster, Margaritifera vulgaris ; the British river mussel, 

 Unio mar garitif eras ; the common oyster, Ostrea edulis, Anodonta 

 cygncea, Pinna nobilis, Mytiliis edulis, Spondylus gcederopus, Area noce, 

 and Anomia ccepa. Similar prominences or concretions occur in the 

 interiors of some porcellanous shells, as for instance in Turbinella 

 scolymus and Strombus gigas, which are of pink colour, but these are 

 not true pearls, not being nacreous, and are said to lose their colour in 

 course of time. 



A popular theory as to the origin of the pearl was that particles of 

 sand were accidentally confined between the mantle and the shell, 



1 Mon. United States Geol. Survey, 1885, vol. ix, p. 79, pi. xiv, fig. 16. 



* Palscontographica, 1890, vol. xxxvi, p. 175, pi. xviii, fig. 6. 

 ^ Trans. Geol. Soc. Glasgow, 1883, vol. vii, pt. i, p. 31. 



* The Geologist, 1859, vol. ii, p. 295. 



^ Quart. Journ. Geol. Soc, 1886, vol. xlii, p. 228; and "The Cretaceous Rocks 

 of Britain (Upper Chalk)," vol. iii, Mem. Geol. Surv. United Kingdom, 1904, 

 pp. 302-48. 



9 I am indebted to Mr. C. D. Sherborn, F.G.S., for calling my attention to 

 Mr. Rose's paper. 



