776 SOME NEW AND RARE HYDROIDA IN THE AUSTRALIAN MUSEUM, 



solid mass of chitinous substance. Sometimes this substance is 

 marked by striae indicating variations of density, and proving its 

 solidity ; but in some cases, as in 0. caliculata, it is homogeneous, 

 so that the hydrotheca resembles an inner cup with an outer 

 calycine envelope, the illusion being, as Mr. Hincks says, so 

 complete that he at first described this as the actual structure. 

 The hydrothecae of Plumularia delicatuta are considerably 

 thickened internally at the same part where in D. mirabilis the 

 calycine envelope is said to exist, and specimens of P. setaceoides 

 in the present collection have an external thickening in front still 

 more extensive. A. productum itself is frequently thickened in 

 the same fashion, if not to the same extent. That this should be the 

 case with some specimens and not with others is quite conformable 

 to experience ; for example, in some forms of C. caliculata there 

 is no noticeable thickening of the calycle, and Dr. von Lendenfeld 

 finds a similar variation in Eucopella. When the chitinous mass 

 is homogeneous in appearance it appears to be very incompletely 

 solidified, so that drying it, or immersing it in a dense medium, 

 such as Canada balsam, causes it immediately to shrink. 



While it would be impossible to decide absolutely, without 

 examination, that the structure of D. mirabilis is as I have 

 suggested, the analogy of the other species mentioned is so strong 

 as to render such a conclusion highly probable, in which case D. 

 mirabilis and A. productum would be clearly referable to the 

 same genus. As regards specific characters, A. productum differs 

 from D. mirabilis in its much smaller size, in the form of the 

 hydrotheca, in the presence of a distinct anterior intrathecal 

 ridge, and in some minor features. 



PLUMULARIA CAMPANULA, Busk. 

 ( = P. laxa, All man ; P. Torresia, von Lendenfeld). 



(Plate XX., figs. 1-6). 



The specimens from the Museum, as well as some from Port 

 Phillip Heads, sent me by Mr. J. B. Wilson, include the gonothecse, 

 which I have hitherto been able to describe very imperfectly, 



