134 AMERICAN HYDROIDS. 



Allman's original description of this genus was as follows: 



"Tr/>li<>xi>n: Hydrocaulus divided into interiiodes, cadi intcniodc carrying a pair of 

 opposite sessile hydrothecffi. 



"Gonosome. -(Jonangia. supported on peduncles which spring from the cavity of certain 

 hydrotlicca', where they take the place of the hydranth." ' 



In his ('/mil, /n/,r Report, the same writer found reason to modify this definition, at least so 

 far as the trophosome is concerned, as follows: 



Hvdrocaulus divided into definite internodes, each internode carrying a pair of opposite 

 hvdrotheea-, or a single hydrotheca which alternates with those of the internodes on each side of 

 it. Hydrotheca' adnate for a greater or less extent to the internode.'" "' 



This genus was recognized by Bale in lss.s :i ami by Marktantier-Turneretscher in is'.to,' 

 who instituted the family Syntheeiidaj for the accommodation of the genus. Torrey takes just 

 the opposite view, and considers the genus untenable, 1 "since it wrenches from their nearest 

 allies such diverse species as Sertularella altemans and Sertularia campylocaTpwn^ and unites 

 them on the basis of a feature which is chiefly interesting to the physiologist." When such a 

 character, however, is associated with others, such as the absence of an operciilum, the smooth 

 margin, the strictly opposite branches, all in a very definite group of species, we. have a perfectly 

 well-deh'ned generic group. The main point of difficulty with the present writer is to determine 

 whether or not this group is of family rather than generic value, as held by Marktanner- 

 Turneretscher. The basing of genera on single characters is a proceeding that is very likely to 

 lead to error, while the grouping of several characters for generic differentiation is a much safer 

 and loo-jcal method. As to the gonosome being chiefly interesting to the physiologist, it must 

 he said that in so far as this character is a morphological feature of great usefulness in classifi- 

 cation, the systematist would be most unwise to relinquish it. 



KEY TO AMERICAN SPECIES OF THK OEM'S .SYNTHECIUM. 



Hydrothcra- opposite. 



llydrothera- tubular, curved in the ijuadrant of a circle, orifice round. 



Each iuternode of stem bear! us; a pair of hydrot IK-CM- tiiliitliu-nin. 



Internodes of stem without, liydrotlu-ca- ft d tun. 



Hydrot IM-CM- doubly curved, orifice triangular marginatum. 



HydrothrcM- short and stout, deeply immersed robustum, 



Hydrot hei-M' alternate I'lilhulrlctim. 



POINTS OF INTER! iRADATION BETWEEN SYNTHEGIUM AND OTHER l.ENERA. 



With .S', rtnliii'iti, in the strictly opposite hydrotheca? and reguhir internodes. The resem- 

 blance to such species as ,S'. flfxmonl, x Torrey is quite close. The gonosome, however, is entirely 

 different. 



With Si'ft tiliii-i/lti. in alternate hydrothec;e, as in <V. cylhttlrlrinii. In this case the entire 

 absence of an operciilum will serve to identify the genus to which a given species belongs. 

 When the gouosome is present there is no trouble of course. 



SYNTHECIUM TUBITHECUM (Allman). 



(Plate XU. fitf. 1.) 



Snlnlnrin liihilln'fii ALI.MA.X, Mem. Mils. Gump. Zool.. V, Xo. L', 1ST7, p. LM. 

 Sertularia tubitheca FKWKES, Hull. Mus. Coinp. /ool., VIII, No. 7, lss|. p. 128. 

 Sertularia tubitheca Nrrnxc;, Nan-alive Hahama Ex]-il.. isn.'i, p. ss. 



Trophosomc. Colony consisting of a monosiphonic stem with strictly opposite liranclies, 

 attaining a height of 1 i inches. Stem cylindrical, divided into regular, long internodes, each of 



'Journal of the Linna-an Society, XII, 1.S7-1, p. l'i;'>. 

 'Challenjier Report, The Hydroida, Pt. L', isss, p. 77. 



"I'rocecdiiiL'S of the biniiM-an Society of New South Wulrs, I'd series, III, ISSS, p. 700. 

 ' Ilydroiilen des k. k. naturhistorischen Hofmuseums, isno, p. ms. 

 llydroida of thi- I'ai-itic Coast of North America, IDOL', p. 02. 



