Limitation of Genera among the Hydroida. 368 



The genus Atractylis, as originally defined by Dr. T. Strethill 

 "Wright, was made to include all those forms of the older genus 

 Eudendrium which are characterized by a fusiform shap'e of the 

 polypite and a conical metastome, the greater number of the species 

 moreover presenting a more or less complete retractility within the 

 summit of the hydrocaulus, though nothing like a proper hydrotheca 

 is ever developed. 



Among the forms, however, which Dr. Wright has included under 

 his genus Atractylis are more than one generic type. One of these 

 types had already been characterized under the name of Perigo- 

 nimus by Sars, who described both the trophosome and the gono- 

 some ; while another had, under the name of Bougainvillia, been 

 long ago described by Lesson, who, however, was only acquainted 

 with the Medusa. That the Bougainvillia of Lesson is the Medusa 

 of a Hydroid form of which the Eudendrium ramosum of VanBeneden 

 {=^ Atractylis ramosa of Wright) maybe taken as the type, has been 

 shown by Dalyell, and confirmed by Wright and others. To this 

 Hydroid and its allied species the name of Bougainvillia must ac- 

 cordingly be restored ; and indeed we find Agassiz already arranging 

 them partly under Lesson's name, and partly under that of Mar- 

 gelis, Steenstrup's name for a Medusa which can scarcely be regarded 

 as different from the Bougainvillia of Lesson. There thus remains 

 only one form of Wright's genus Atractylis which had not already 

 received a distinguishing generic designation, — that, namely, which 

 is represented by the Atractylis arenosa of Alder, whose gonosome 

 has been recently so well described by Dr. Wright (Micr. Journ. 

 n. s. vol. iii.). To this form, therefore, it will be necessary hence- 

 forth to restrict the name Atractylis. 



Atractylis arenosa, Alder. 



The following species cannot be regarded as otherwise than 

 provisionally referred to the genus Atractylis. Two of them 

 will probably turn out, when the gonophores shall have been 

 observed, to belong really to the genus Perigonimus ; while a 

 third is undoubtedly the type of an entirely new genus. 



Atractylis coccinea, Wright. 



Atractylis miniata, Wright. 



Atractylis margarica, Hincks. 



The Atractylis margarica has been described by Mr. Hincks 

 in the 'Ann. and Mag. of Nat. Hist.' for January 1863. It is cer- 

 tainly not an Atractylis, but is the type of an entirely new genus. 

 I refrain, however, from giving here a definite form and name to the 

 genus which I know must be constituted for it, preferring to leave 

 this duty in the hands of its discoverer, Mr. Hincks, who, I have 

 little doubt, will take the same view in his forthcoming work on the 

 Hydroida. 



3. BiMERiA, Strethill Wright. 

 Trophosome consisting of a branching rooted coenosara in- 



24* 



