THE PLUMULAKIDvE. 



81 



Distribution. Aalesund and Lot'ot, Norway, 50 to 200 fatlioms; New Knglaud coast, Verrill 

 and Fewkes; shallow water. 



This elegant little species was the second of the Eleutheroplea found on the New England 

 coast. Specimens kindly sent me by Professor Verrill are not separable, either in trophosotne or 

 gonosome, from 7'. </>-aciUinia Sars, and it therefore becomes necessary to deprive the species of 

 the honored name which it bore, and adopt the very appropriate name proposed by its original 

 describer. The species is clearly a tichizotriclui if we adopt that genus as described by Allmaii 

 and further defined in this work. 



Typical specimens in the museum of Yale University. 



DIPLOPTERON Allman (in part). 

 Diplopteron ALLMAN, Report <m Hydroida of Porcupine, 1874, p. 479. 



Trophosome. Two or more pairs of lateral neuiatophores flanking the hydrothecpe. 



Gonosome. Gonaiigia protected by accessory ramuli borne on the hydrocladia, and bearing a 

 few scattered liydrothecre. 



The first species described as having two pairs of lateral nematophores was Diplopteron 

 iitxif/nc, the type species of this genus, and I here adopt the suggestion made by W. M. Bale: 1 

 "If this genus (Diplopteron} be retained, I would suggest that it be modified so as to comprise 

 all species with more than one pair of lateral nematophores." - The present writer would not, 

 however, follow the further suggestion that /'. uijluoplienoMes be admitted to this genus. The 

 fixed mesial nematophore of that species is a character which, in the opinion of the author, 

 is of considerable systematic importance, at least of generic rank. 



The genus Poli/iilinin/liiriK Sars, in favor of which Allman in the Challengn- report abandons 

 his genus Diplopteron, was founded on the bipinnate arrangement of the hydrocladia. Diplopteron. 

 on the contrary, was established upon an entirely different basis, namely, the two pairs of lateral 

 nematophores. It thus becomes proper to retain the genus, regarding it as distinct from 

 Potyplumularia. 



As here denned the genus Ih'ploptrnm would include />. inxigne and the three new species 

 described below, which differ from />. inxiijne in the almost entire absence of hydrocladial iuternodes, 

 in which they agree with Hcliizulrivlm ilicliotoma. The three genera, Hrltizofrirlm, Potyplumularia, 

 and Diplopteron, are very closely allied, and may ultimately be united by the discovery of completely 

 intergrading forms. The author believes that generic distinctions, like specific characters, are 

 based rather upon our ignorance than our knowledge, and that with a complete series of almost 

 any genus or species these distinctions would fail, making it necessary to resort to arbitrary 

 definitions for the sake of convenience in handling groups. It is almost certain that naturalists 

 will in time realize that genera and species are not entities or facts, but convenient fictions useful 

 in discussion, indeed necessary in systematic work, but not really tangible. 



Distribution <>f the American species of the genus Diplopteron. 



KEY TO AMERICAN SPECIES OK IJIPLOPTERON. 



One pair of lateral nematophores greatly elongated D. quadrioorne. 



Both pairs of lateral nematophorea normal : 



Hydrothee.-i' very large, anterior protile doubly curved U. ijrtinile. 



Hydrothecre much smaller, anterior profile with a single curve />. longipinna, 



1 Catalogue of Australian Hydroid Zoophytes, p. 124. 



'- This author subsequently abandons the genus Diplopteron (see The (.ieiiera of I'liimnlariidu;, 1886), but I consider 

 the genus tenable on the grounds suggested above. 



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