106 AMERICAN HYDROIDS. 



irregular, but usually with a quadrilateral outline, with the corners of the quadrilateral very 

 slightly, if at all produced into four very low obscure teeth; operculum with four Haps. 



Gronosome. Gonangia borne in bifurcations of the branches, very large, ovate, body with 

 shallow broad obscure annulations; neck in the form of a long truncated cone with a round 

 terminal aperture. 



DitstrHmtluii.AnH-ttriiKx Station 2842, lat. N. 54 15', long. W. 166 3', 72 fathoms; Station 

 2874, lat. N. 48 30', long. W. 124 57', 27 fathoms. 



Type xlklcx. Cat. No. 19780, U.S.N.M.; Cat. No. Is721, Museum of State University of 

 Iowa; also in the collection of the author. 



DIPHASIA Agassiz (modified). 



Troplioxonu'. Hydrotheca? biserial, opposite or alternate, aperture broad, operculum evident, 

 of a single adcauline flap. 



Gonosonu'. Gonangia usually differing in the sexes, and marked with spines or lobes; an 

 internal marsupium usually present in the female. 



This genus as proposed originally by Louis Agassiz 1 was very inadequately characterized, 

 the only definition being in a footnote, as follows: "In the genus Di]>Ii<ixin the fertile hydra? 

 are deeply dentated." Hincks, in his British Hydroid Zoophytes, 18H8, insists that the main 

 feature is the marsupial chamber of the female gonangium, in which he is followed by Bale. 2 

 Four years later Allman 3 called attention to the important character of the lid-like operculum 

 which is more conspicuous and constant in this than in any other genus of the family. Kirchen- 

 pauer practically adopted Allman's definition. 4 All of these writers considered the marsupial 

 chamber in the female gonangium as a necessary character of the genus. Levinsen 5 claims that 

 this character is not constant, and occurs also in other genera, and liases his diagnosis of the 

 genus on the characters of the margin and operculum alone, thus including all of the species of 

 Abietinaria as used in this work. My own opinion is that Ab!<.'t!iir!<i is a good genus, based on 

 the shape of the hydrotheca 3 , and can very well be differentiated from Diplutsnt on that character, 

 there being but one form, at least among American species, that cannot readily be relegated to 

 one or the other of these genera, and that is D. piilcltni Nutting, which in general texture is 

 more closely allied to Dipluu<!a, which usuallj* lacks the rigid clear-cut sturdy hydrothecal outline 

 that appears to be characteristic of Aliletlnarni. 



KEY TO AMERICAN SPECIES OP DIPHASIA. 



Hydrotheca? in strictly opposite pairs borne on sides of hydrocaulus. 



Margin sinuous, but not toothed, hydrothec.t not regularly annulated. 



About one- third of hydrotheca free rosacea. 



Less than one-third of hydrothecre free, aperture very wide j'<i//<i.<: 



Margin with three teeth, hydrotheca; slender, tubular tamarisca. 



Margin not toothed, hydrothecal walls regularly annulated Im/iicn. 



Hydrotheca; in opposite pairs, borne on front of hydrocaulus , digitalis. 



Hydrothecie not strictly opposite, at least on branches. 



Branches arising from all sides of stem, forming a spiral piilrhrn. 



Branching pinnate. 



Each internode of stem bearing a pair of opposite, hydrotheiw ]ianr>iumni. 



More than two hydrothecjp to eacn internode. 



Gonangia wit h two or more lateral spines corniculata. 



Gonangia wit hout spines kincaUli. 



Contributions to the Natural History of the United States, IV, 1802, p. 355. 



2 Australian Hydroid Zoophytes, Sydney, 1884, p. 98. 



"Challenger Report, Hydroida, Ft. 2, London, 1888, p. 63. 



4 Hydroiden des k. k. naturhistorischen Hofmuseums, 1890, p. 237. 



''Meduser, Ctenophorer og Hydroider fra Groiilauds Vestkyst, Copenhagen, 1893, p. 196. 



