323 PERIGONIMUS MUSCOIDES. 



Tliis position of the gonophores, however, we now know to be equally a character of other 

 genera, while still other characters of a specific rather than a generic value were also included by 

 Savs in his diagnosis. Yet notwithstanding this, the hydroid on which the genus Perigonimus 

 was founded is undoubtedly entitled to generic rank, and, with some shght modifications of the 

 original diagnosis, Perigonimus must stand as an established genus. 



The genus Atradi/lis of Wright, as already mentioned,^ includes forms which are referable 

 to the T criyonimus of Sars, and others which must l)e transferred to the BougainviUia of Lesson, 

 while, cjuite recently. Van Beneden" has instituted his genus Dinema for a form which cannot be 

 generically separated from Perigonimus. 



The only character on which the claims of Binema to be separated from Perigonimus I'est 

 are found iu the fact that at the time of liberation the medusa of Dincma has only two marginal 

 tentacles, while that of the Perigonimus muscoides of Sars has four. This, however, is a very 

 unimportant difference, for there can be no doubt that the tentacles increase in number with the 

 age of the medusa, and that the two-tentacled Binema acquires four or more tentacles as it 

 advances towards maturity. I have accordingly already united^ under the genus Perigonimus, 

 not only forms in which the medusa, at the time of its liberation from the trophosome, has four 

 tentacles, but those in which the number of tentacles at that period does not exceed two. The 

 changes, indeed, which occur in the medusa as it approaches sexual matMity are to be chiefly 

 sought for in the increase of the number of marginal tentacles, each new one being intercalated 

 at the middle point between two older ones. 



*^* 1. Peeigonbius muscoides, Sars. 



Pekigonimus muscoides, — Sars, Fauna Lit. Norv., crste Lieferung, p. 8, tab. i, figs. 



19—21. 



TROPHOSOME. — Hybrocaulus attaining a height of from two to three inches, 

 much branched, the branches thinner than the main stem, and, as well as the main 

 stem, sending off numerous short, scattered, simple hydranth - bearing ramuli ; 

 PERiSAKC marked with longitudinal, somewhat undulating strias, not annulated. 

 Hydranths with a circlet of from eight to twelve tentacles in two closely approxi- 

 mated series. 



GONOSOME. — GoNOPHOKES borne on short peduncles and scattered over the 

 main stem and principal branches. Medusa, on liberation, with four marginal 

 tentacles. 



1 See p. 299. 



' ' Recherches sur la Faune litt. de Belgique,' p. 127, pi. i.K and x. 



' " Ou tlio Coustructiuti and Limitation of Genera," S:c., ' Ann. Nat. Hist.' for May, 1861. 



