MancJiestey Memoirs, Vol. xlviii. (1903), No. 4. 5 



The prostate gland like the vesicula is usually provided 

 with well-developed muscular walls. In some genera it 

 is lost completely. Interesting stages in its reduction can 

 be traced in the genus Leptoplana, whilst it has altogether 

 disappeared in Discocelis, Planctoplana ('92) and in other 

 genera of the Acotylea, and in the Diposthiidae and Diplo- 

 pharyngeatidae amongst Cotylean forms. In some cases 

 e.g.., Discocelis, the prostate is replaced by glands of a 

 prostatic character which appear on the walls of the 

 antrum. These glands in Thalamoplana}^ are situated at 

 the ends of definite muscular projections from the walls 

 of the antrum masculinum, which are of great interest, 

 since they show how the extraordinary ' intromittent 

 prostate' of the Diposthiidae may have originated. In 

 most of the Cotylea the walls of the antrum masculinum 

 are folded so as to form a special penis sheath. Amongst 

 the Acotylea this sheath occurs only in the Cestoplanidae. 



The characters discussed above, taken in conjunction 

 with others of importance, such as the position of the 

 tentacles or tentacle eye-groups, the presence or absence 

 of marginal eye-spots, and the characters of the pharynx, 

 have enabled me to draw up the following diagnostic 

 table of the majority of the genera included in the division 

 Acotylea. In order to make the table more complete, I 

 have included in it the Cotylean families. 



I have omitted altogether the genera Cryptocelides 

 ('93), Polypostia ('93), and Bergendalia (:03«) because, 

 whilst their peculiarities enable them to be readily dis- 

 tinguished, they make it impossible to define their relation- 



*A detailed account of this new genus will be published in Prof. 

 Herdman's Report on the Pearl fisheries of Ceylon, now in the press. 

 The genus may very briefly be defined as follows : — A genus closely allied 

 to Discocelis, but with the genital apertures separated. The prostatic glands 

 are carried on muscular projections from the walls of the antrum masculinum. 

 Accessory vesicle crescentic. 



