196 CEYLON PEARL OYSTER REPORT. 



from the rest of the Turbinolidse, and placing them in a separate family, Flabellidae, 

 and the septal arrangements and the peculiar mode of multiplication by transverse 

 fission, characteristic of the family, are additional reasons for keeping it apart. 

 Gardiner (23) has shown that the genera Blastotrochus, M. Edw. and H., and 

 Rhizotrochus, M. Edw. and H., must be absorbed. I have not been able to examine 

 Duncan's fossil genus Thysanus, and I do not include it in the family Flabellidse, as 

 here defined, because it has well developed granular and minutely spined costse, 

 which seem to indicate an external thickening of the wall. 



The genus Placotroehus was not included by Duncan (13) in his alliance Flabel- 

 loida, but was placed, along with Sphenotrochus, Nototrochus, Plaeoeyathvs, and 

 Platytrochus in an alliance Placotrochoida, characterised by the presence of an 

 essential lamellar columella. For this I can find no justification whatever. Placo- 

 trochus has no edge-zone ; its wall is protothecal and devoid of costas ; it has the 

 compressed flabelliform or cuneiform shape characteristic of Flabellum ; it reproduces 

 itself asexually by transverse fission from a nurse-stock, and the truncated free forms 

 have a basal scar exactly like that of Flabellum ; it has protothecal spines ; its septal 

 arrangements are those of Flabellum, The only point of difference is the essential 

 lamellar columella, but this cannot outweigh the other characters, and the well- 

 preserved specimens of this genus in Professor Herdman's collection show that the 

 general anatomy of the polyp is the same as that of Flabellum, though 1 have 

 not yet completed my study of its microscopic structure. I have no hesitation in 

 placing it in the family Flabellidse. 



Flabellum, Lesson (1831). 



A detailed criticism of this genus has recently been given by Gardiner (22), to 

 which the reader is referred. The subdivisions Subpedieellata truncata and Ji.ra, of 

 M. Edwards and Haime, were shown by Semper (49) to be purely artificial, and 

 Duncan's subdivisions of the genus (13, p. 13) are still more arbitrary and unnatural. 

 Semper, and more particularly Gardiner, have shown that most of the living species 

 enumerated in the ' Hist. Nat. des Coralliaires,' must be regarded as varieties of a 

 few distinct species, and the latter author has reduced most of M. Edw. and Haime's, 

 as well as Semper's species to varieties of either F. pavoninum, Lesson, or F. rubrum, 

 Quoy and Gaimard, the latter sj)ecies being apparently protean in its characters. I 

 speak with all diffidence, for I have not had the opportunity of comparing a large 

 number of specimens, but I am inclined to think that Gardiner has gone too far. 



Flabellum crassum, M. Edw. and H. Plate I., figs. 3 and 3a. 



There are in Professor Herdman's collection two specimens of Flabellum, from the 

 pearl banks in the Gulf of Manaar, which agree in almost every respect with Semper's 

 description and figures of F. irregulare. Both specimens were preserved in spirit ; 

 one I have decalcified, the corallum of the other when cleaned and dried measures 



