REPORT ON THE HYDROIDA. Bi 
groups of closely approximated plumes. The mesial nematophore is provided with an 
imperfect septum near its distal extremity. The most remarkable fact, however, connected 
with the species is the occurrence of two different forms of corbula 
a closed and an open 
—in the same colony. The closed corbula is rather short, and with its rachis so curved as 
to give it a somewhat crescentic form, with the concavity looking downwards. The 
curvature of the open corbula is less marked. In the open form the coste are entirely 
distinct from one another ; they are oval in outline, each margin set with tubular denticles, 
and every costa having a tubular spur-like denticle at its base. On the proximal margin 
of every costa in the open corbula, and close to its origin, is a slit-like aperture which leads 
into its cavity. 
T am unable to assign a meaning to the presence of two different kinds of corbula in 
one and the same colony, and I do not know of a similar occurrence in any other species. 
That the two kinds of corbula are not different stages in the development of a single form 
would appear from their similarity in size, and the apparently mature condition of both, 
with their thick chitinous periderm ; nor is it likely that at the time of maturity the closed 
corbula had become converted into the very differently formed open one by a process of 
dehiscence. Though in none of the open corbule could I find gonangia, it is yet possible 
that the difference may indicate a difference of sex. It is also possible that the open form 
may be an abnormal and imperfect condition of the corbula, not essential to the species, 
or universally occurring in it. With this uncertainty I have deemed it better not to 
include the presence of two forms of corbula as an essential character in the diagnosis of 
the species. 
Aglaophenia filicula is an inhabitant of the deeper sea zones, having been dredged 
along with Aglaophenia acacia, at Station 75, July 2, 1873, lat. 38° 37’ N., long. 28° 
30’ W.; depth, 450 fathoms ; bottom, sandy. 
Aglaophenia attenuata, n. sp. (Pl. XI. figs. 7-9). 
Trophosome.—Colony attaining a height of about two inches ; stem slender, monosi- 
phonic, slightly and irregularly branched ; hydrocladia very short, scarcely exceeding one- 
twentieth of an inch in length, alternate. Hydrothecze closely approximate, deep, margin 
deeply dentate, with the mesial tooth bifid, anterior wall depressed just below the margin ; 
intrathecal ridge well marked, extending transversely across the hydrotheca, at the junction 
of the lower and middle third of its walls ; mesial nematophore stout, adnate to the walls 
of the hydrotheca for somewhat more than half their height, and then extending asa 
short, thick, free beak, which does not reach the margin of the hydrotheea ; lateral nemato- 
phores stout, reaching the level of the hydrotheca margin. 
Gonosome.—Corbula open, with about seven pairs of costa, which are quite distinct 
from one another, and carry a row of tubular denticles on each margin ; rachis with a 
spur-like denticle at the base of each costa, 
