244 CEYLON PEARL OYSTER REPORT. 



chambers of Jletcropsammia and Heterocyathus, and states that it was referred by 

 Mr. E. A. SMITH to ANGAS' genus Mysella. 



Among the solitary corals from Ceylon sent me by Professor HERDMAN were 

 numerous spirit-preserved specimens of Heteropsammia michelini and Heterocyathus 

 (Rjuicostatus, and on opening the Aspidosiphon chamber in one of these I was at once 

 struck by the presence of the numerous small Lamellibranchs, many of them imbedded 

 in the skin of the posterior part of the body of the Sipunculid, as described by 



BOUVIER ; others lying free in the innermost coils of the chamber, especially in its 



j & |- .,-. 



terminal part. 



Having many specimens of the corals at my disposal, I examined a large number 

 of them and invariably found a number of the Lamellibranchs inhabiting the 

 Aspidosiphon chamber/ In some of the larger specimens of Heteropsammia I found 

 as many as 30 or 35 specimens of different ages, in some of the smaller specimens of 

 Heterocyathus not more than a dozen or fifteen. 



BOUVIER left it an open question whether the commensal Mollusc was an adult 

 form or not. My observations quickly showed me that a proportion of the specimens 

 inhabiting each coral were adult, and that along with them were numerous young 

 forms in all stages of growth. With the abundant material at my disposal I 

 proceeded to make a careful study of the anatomy, and, as far as the circumstances 

 allowed, the development of this hitherto undescribed species, and although but few 

 out of the many specimens were sufficiently well preserved to admit of satisfactory 

 microscopical examination, I found a sufficient number in good enough condition to 

 enable me to work out the structure in some detail. 



i .'. .,.'.''. i '.'"!' i[ii . 



DESCRIPTION OF THE GENUS AND SPECIES. 



The description of the genus Mysella given by ANGAS (1) is based on the characters 

 of the shell only, and his figure of the hinge apparatus is so small that it is difficult 

 to make out the characters of the hinge teeth clearly, but it is evident that the 

 species commensal in the two corals cannot be referred to his genus. 



The specimen on which the genus Mysella was founded was 7'5 niillims. in length, 

 and was found in black mud near Port Jackson. The shell is inequilateral, the 

 anterior side being the shorter and subtruncate ; judging by the figure, the umbones 

 are prosogyrous. The ligament is internal, and there is a single small diverging 

 aubcircular flattened cardinal tooth in one valve, and two short thin horizontal 

 lateral processes in the other valve. 



My specimens agree with Mysella in having an internal ligament, and in having a 

 single cardinal tooth in the right valve and two teeth in the left valve ; but there are 

 in addition well-developed lateral teeth, and the shape of the shell is quite different. 

 Moreover, the largest of my specimens does not exceed 1-5 niillims. in length and the 

 average length of the adult forms is I'O millim. 



