tisnally refcracteil within a muscular collar, which possesses 

 a denticulated annulus. Averaji^e diameter 0*78 mm. ; 

 musculature of the myzorhynchus showing no trace of 

 division into bundles; collar-sheath usually widely open in 

 the resting stage. 



Resting in fibrous connective-tissue cysts in the tissues of 

 Maryaritifera vulgaris from the Gulf of M;i,naar. 



Tylgcephalum minus, sp. n. 



Similar to the above but smaller, average diameter 0*14 

 mm. As a rule the myzorhynchus appears more conical 

 than in T. liidijicans, and the collar-sheath more constricted, 

 with smaller aperture, in the resting stage. In some 

 examples the musculature of the myzorhynchus shows a 

 tendency to bi-eak up into bundles of tibres. 



Habitat as in T. ludijicans. 



Dr. Jameson claimed that he had found Tylocephalmn ludificans 

 in specimens of the Ceylon Pearl-Oyster in Dr. Kellart's collection 

 in the British Museum, and had considei-ed the possibility of 

 their being concerned with peaid production, and dismissed the 

 theory as untenable, previously to Prof. Herdman's original 

 departure for Ceylon. 



The second part of the paper dealt with the structure and 

 formation of the shell and of per.rls. The various repair- 

 substances, which replace the ordinary shell substances under 

 abnormal or pathological conditions, were described, their relations 

 to the normal substances of the shell were discussed, and theii- 

 occurrence in the pseudo-nuclei of pearls dealt with. The 

 " calcospherules " which Herdman regarded as free concretions, 

 and as the cause of " Muscle Pearls," were considered to be in 

 fact minute pearls, composed of the hypostracum, or special shell- 

 substance to which the muscles are attached. This conclusion 

 had been reached independently by Rubbel, in Marburg. 



The author said that his observations on the real cause and 

 mechanism which led to the formation of pearls in the Ceylon 

 Pearl-Oyster were still too incomplete to communicate ; but he 

 maintained that, as he had already laid down in his 1902 papei-, 

 the real cause of pearl production would have to be sought, not 

 in the nuclei or pseudo-nuclei of pearls, but rather in the patho- 

 logical conditions under which the tissues of the mollusc gave 

 rise to the pearl-sac. It was only in a few cases, like the 

 Ti-ematode pearls in the common mussel, that the cause of the 

 pearl-sac, i. e. in this case the trematode, frequently remained to 

 form the " nucleus" of the pearl and tell the tale of its origin. 



The author had found that, as observed by Prof. Herdman, a 

 minority of Ceylon pearls may have foreign bodies, such as sand- 

 brains, in their centres. 



