EEPOET OjST THE HYDEOIDA. 
51 
forming when in apposition a four-sided or three-sided pyramidal roof for the hydrotheca. 
In some cases ( e.g ., Sertulciria clistans ) there are but two valves, and these when in 
apposition form a wedge-shaped roof over the orifice of the hydrotheca. 
The valves in all these cases are so thin and perishable that it is only in recent or 
exceptionally well preserved specimens we can hope to meet with them, a fact which in 
itself deprives the distinctions derived from them of that practical value which ought if 
possible to be found in all well-selected systematic characters. 
On the grounds here adduced, then, I must regard the removal from Sertulciria of the 
species for which Gray founded his genus Sertularella as a step not borne out by funda- 
mental differences in the species ; and although Gray’s revision has been adopted by so 
high an authority as Hincks, and though I had followed it myself in former publications, I 
shall in the present Report suppress the genus Sertularella and refer to Sertularia all 
species which agree with the diagnosis of this genus as given above. 
A revision much more in accordance with natural affinities was proposed by Agassiz, 
who, under the name of Diphasia separated from Sertularia those species in which the 
female gonangium differs from the male in possessing an external chamber with chitinous 
walls, into which the ova are expelled from the main cavity of the gonangium in order to 
pass through certain stages of their development before escaping into the surrounding 
water. 
Sertularia gracilis, n. sp. (PI. XXIV. figs. 1, la). 
Trophosome. — Stem monosiphonic, slender, profusely branched ; primary ramifica- 
tion pinnate or subpinnate, many of the primary and secondary branches presenting 
a similarly pinnate or subpinnate ramification. Hydrothecae alternate, distant, one 
borne on every internocle, nearly cylindrical, aclnate to the internode for about two- 
thirds of their height, then bending outwards ; orifice with a broad cusp on each side. 
Gonosome . — Gonangia springing each from a point just below the base of a hydro- 
theca, urniform, with very prominent annular ridges, distal end continued as a narrow 
cylindrical tube, which carries the even, circular orifice on its summit. 
Locality . — Station 312, Port Famine, Patagonia; lat. 53° 37' 30" S., long. 70° 56' 0" 
W.; depth, 9 fathoms. 
This slender and very elegant species was brought up in luxuriant masses about five 
inches in height, and richly laden with gonangia. The ramification, which is very 
profuse, commences with pinnately disposed branches, many of which soon branch in a 
similar way, and the primary ramification is further repeated in ramuli of a secondary, 
tertiary, and even higher order. The branches are given off each close to the base of a 
hydrotheca. 
