38 AMERICAN HYDKOIDS. 



and in having a well-marked operculum consisting of four segments. On the other hand, we find 

 in some species of Selaginopgis a superficial resemblance to certain of the Perisiphouidse and 

 Lafoeida-. both of which families have the hydrotheca- without pedicels and arranged on all sides 

 of the hydrocaulus, but which differ from the Sertularidse in having the stem and usually the 

 branches composed of a number of parallel tubes. 



The genus TTyiJi-nUinfiinn^ as indicated above, bears some resemblance to the Plumularida 1 

 in having its hydrothecse arranged in a linear series on the upper sides of the branches, but differs 

 from all known plumularians in having the distal ends of the hydrothecse bent alternately to the 

 right and left, as well as in the absence of nematophores. 



Although the family itself seems to be sufficiently well characterized, almost insuperable 

 difficulties are encountered when we attempt to break it up into genera. Various writers have 

 offered solutions which seem well conceived when the material at hand is limited, but which 

 break down more or less completely when all material available from more recent explorations is 

 taken into consideration. Perhaps the difficulties encountered in trying to solve this exceedingly 

 perplexing problem can best be demonstrated by a brief summary of the attempts made by the 

 more important authorities, beginning with Hincks's epoch-making work. British llydroid 

 Zoophytes, published in 186)$. This writer divides the Sertulariidae into the following genera: 1 



n. Hydrothee.'e biserial, decidedly alternate, operculum composed of several pieces. 



'isiit. Hydrothecse opposite, occasionally subalternate, a pair to each intercede, with an internal valve-like 

 operculum. Female gonangium with an internal marsupial chamber. 



Sci-lnliiri<i. Hydrotheca; biserial, opposite or alternate, without external operculum. Gonangia without internal 

 marsupium. 



Tlmiftria. Hydrothecae biserial, embedded in the substance of the stem and branches. 



All of these genera, modified to accommodate themselves to the results of later investiga- 

 tions, are still retained by recent writers and in the present work. 



In December, lsT4, Professor Allman read a paper before the Linna-an Society.- in which 

 he defined the following new genera: 



]>i_'sintirii/,lmx. Hydrocaulus bearing hydroth.ec.fe which are adnate to each other in pairs, and each pair adnate 

 to the front of the hydrocaulus. 



Synthrciuiii. Each iutemode of the hydrocaulus bearing a pair of opposite sessile hydrothecse. Gonangia on 

 peduncles springing from within the cavity of hydrotheae. 



Kilni/iiKijisix. Hydrotluv;i- disposed in several longitudinal rows about the nonfascicled hydrocaulus. 



Perirladiuin. Hydrothecse more or less immersed and closely set amund bifurcating ramuli which .-prim: from 

 the sides of a common stem. 



Iii this work Professor Allman separates the genus Tlmiin-in from the Sertuhirida 1 and 

 places it in a new family, the Thuiariida>. 



In his report on the Hydroida collected by the Challenger* the same author modifies the 

 genus X rfiifiir/it so as to include the genus X, rf />////< H</, modifies the genus Thnlurla and restores 

 it to the Sertularida', and forms three new genera. His classification is as follows: 



.s. rtidaria. Hydrothecae sessile, in two series, opposite or alternate, margins entire or dentate, sides more or less 

 adnate to the hydrocaulus, an internode to each two hydrotheca?. Operculum present or absent. 



Tliuinrin. Internodes of hydrocaulus each bearing many hydrothecse which are alternate or opposite, more or 

 less adnate to hydrocaulus, margin entire or dentate. 



iixin. Substantially as defined by Agassi/ and Ilincks. 



ix, Siintliii'iinii, and Tliei-nelmHuni. As defined above. 



i/.iix. Like ]>i:iiiin.ii-i/iilius, but with two minute cup-shaped appendages (nematophores?) at the base of 

 each hydrotheca. 



rn. Hydrocaulus with opposite hydrothec;c arranged in decussating pairs. 



ii. Hydrocaulus forming a flabelliform network of anastomosing steins and branches. Hydro- 

 on all siiles of I>ranches. 



'British Hydroid Zoophytes, London, 1868, ]i. L'iU c( xer/. The above table is not a quotation direct, but a 

 condensed statement of the most important points in his definitions of genera. 

 2 Linna?an Society Journal, Zoology, XII, 1S76, p. 252 et seg. 

 'Report on the Hydroida, Part 2, 1888, pp. 49, 50. 



