REPORT ON THE HYDROIDA. 35 



sending off closely-set opposite primary pinnae, which are destitute of hydrothoero, 1iut 

 carry from end to end the proper h3'-drotheca-l3earing pinna), which are about one-tenth of 

 an inch in length, alternately disposed, and borne also by the main stem in the intervals of 

 the primary pinnje. Hydrothecae closely approximated, rather deep, with a well-marked 

 intrathecal ridge, margin entire, sigmoidally curved ; mesial nematophore adnate by its 

 whole length to the hydrotheca, with its sides parallel in profile, diverging from below 

 upwards in front view, its distal end terminating close to the margin of the hydrotheca 

 by a wide orifice, a short distance l)ehind which is a transverse septum-like fold ; lateral 

 nematophores wide, overtopping the hydrotheca ; hydrotheca! internode with strong 

 obliquely-directed intra-cauline ridges. 



Gonosome. — CostiB of phylactocarp intimately fused together, so as to form a long- 

 nearly cylindrical, completely closed corl^ula, whose sides support seven or eight trans- 

 verse series of slightly elevated ramified ridges, with every branch supporting a wide 

 cup-shaped nematophore, and with a strong spur-like nematophore at the Ixase of each 

 series. 



Aglaojikenia macgiUivrayi presents many features in which it differs from the typical 

 AglaojjJienm. Its doubly pinnate ramification affords in itself a character which contrasts 

 markedly with the ramification of nearly all the other known species. This character 

 shows itself in the thick, strongly-fascicled stem being closely set with opposite pinnse, 

 which are themselves destitute of hydrothecse, but give support to the proper hydrocladia, 

 which arc disposed in alternate pinnas from end to end of the opposite or primary pinnae. 

 Here and there the primary pinnae become developed into long branches, which then re- 

 peat in all respects the ramification of the stem. The absence of true serration in the 

 margin of the hydrotheca, and the bithalamic condition conferred on the mesial nemato- 

 phore 1 >y the presence of an imperfect septum, are other important points in which Aglao- 

 phenia macgiUivrayi differs from the typical AgJaopheime ; while in the corliuln, witli its 

 parietal ramifying ridges, we find a still further distinctive feature. 



A comparison of the Plumularidan here described with authentic specimens of Plumti- 

 laria macgiUivrayi, Busk, brought home by the " Eattlesnake,"^ establishes the identity of 

 the " Rattlesnake " species with that of the Challengei'. Kirchenpauer places it in his 

 .special section or sub-genus of Aglaophcnia, to which he gives the name of Pachy- 

 rhyncliia, and which he bases on the supposed excessive width of the mesial nemato- 

 phore, believing the nematophore to surpass in this respect the hydrotheca to which 

 it belongs.^ I am unable, however, to accept this character as a ground for the establish- 

 ment of a distinct group, more especially as its reality is not borne out by an examination 

 of well-preserved specimens, and Kirchenpauer's figures seem to have been made from 

 specimens whose bad state of preservation rendered them deceptive. The mesial 



1 Voyage of H.M.S. "Rattlesnake," vol. i. p. 400. 

 - Kirchenpauer, loc. cit., p. 20. 



