REPORT ON THE HYDROIDA. 37 



groups of closely approximated plumes. The mesial nematopliore is provided with an 

 imperfect septum near its distal extremity. The most remarkal)le fact, however, connected 

 with the species is the occurrence of two different forms of corlnila- — a closed and an open 

 — in the same colony. The closed corhula 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 corbulo, is less marked. In the open form the costse 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. 



I am unable to assign a meaning to the presence of two diflferent 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 1ioth, 

 with their thick chitinous periderm ; nor is it likely that at the time of maturity the closed 

 corbula had Ijecome converted into the very differently formed open one by a process of 

 dehiscence. Though in none of the open corljulee 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 liaA'^e deemed it better not to 

 include the presence of two forms of corbula as an essential character in the diagnosis of 

 the species. 



AglaopJienia Jilicula 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 fixthoms ; bottom, sandy. 



Aglaophenia attenuata, n. sp. (PI. 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. Hydrothecse 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 hydrotlieca, at the junction 

 of the lower and middle third of its walls ; mesial nematopliore stout, adnate to the walls 

 of the hydrotheca for somewhat more than half their height, and then extending as a 

 short, thick, free beak, which does not reach the margin of the hydrotheca ; lateral nemato- 

 phores stout, reaching the level of the hydrotheca margin. 



Gonosome. — Corbula open, with about seven pairs of costae, 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. 



