Mr. E, Forbes on the British KcWxnuAse.. 181 



In Actinia, (1.) the arrangement of the tentacula; (2.) the 

 structure of the oral disk; and (3.) the shape of the body. 



As there is only one species as yet known of each of the 

 genera Adamsia and Iluanthos, it is impossible to say cer- 

 tainly what are the points of specific character in those ge- 

 nera. Probably they will depend in the first on the tentacula 

 and colouring ; in the second on the tentacula and sulcature 

 of the body. In assigning sources of specific character I have 

 been guided by the analogies of the genera, taking it as o. pro- 

 bable law, that the points of specific character correspond in 

 animals at once analogous and alUed, and that the points of 

 specific character in the typical genus of a tribe are mainly 

 characteristic of the tribe itself. 



II. The genera of British Actiniadae may be essentially 

 characterized as follows : — 



I. Anthea (Johnston). Body cylindrical, adhering by a 

 broad base. Tentacula simple, non-rectractile, surround- 

 ing the mouth. 



II. Actinia (Linnaeus). 5ocfy cylindrical, adhering by a 

 broad base. Tentacula simple, retractile, surrounding 

 the mouth. 



III. Adamsia (Forbes). Body expanded, bilobed, adhe- 

 ring by a broad base. Tentacula subretractile, simple, 

 surrounding the mouth. 



IV. Iluanthos (Forbes). Body cylindrical, tapering to 

 a point at its posterior extremity, free ? Tentacula sim- 

 ple, retractile, surrounding the mouth. 



V. Lucernabia (Muller). Body campanulate, adhering 

 by a narrow base. Tentacula in tufts at regular distances 

 on the oral margin. 



As among zoophytes anatomical characters are of secondary 

 generic, though of primary ordinal importance, I have not 

 reckoned them essential. The two last genera may however 

 be distinguished anatomically from the other three by their 

 converging ovaries. This organization is the result, not the 

 cause, of external form. In drawing up the above generic cha- 

 racters, I have borne in mind the existence of single Helian- 

 thoid Polypes wanting tentacula or with branched tentacula. 



