148 PEOr. ALIiMAK ON NEW HTDEOIDA PEOM 



great extent to which the hydrothecse are adnata to the inter- 

 nodes, but in the disposition of the joints by which numerous 

 hydrothecse are borne on each side of a single internode. The 

 same condition is presented by many of the pinnae, each inter- 

 node carrying two or three pairs of hydrothecse which are quite 

 separate from one another, but adnate in their entire height to 

 the sides of the internode. 



In other pinnse, however, the hydrothecse of each pair are 

 brought to one side of the supporting internode, and instead of 

 being distinct, are closely approximate and adnate to one another 

 by their opposed sides. This is the essential character of the 

 genus Desmoscyphus, wbich is thus distinguished from Thuiaria. 

 Further, many pinnse which are typically Thuiarian in their 

 proximal portion, lose all trace of Thuiarian characters towards 

 their distal extremities, and here carry on each internode a single 

 pair of distinct hydrothecse. These iuternodes with their hydro- 

 thecse are in every respect those of a typical Sertularia. The 

 orifice, moreover, instead of being nearly circular and even and 

 directed away from the supporting internode, as in the other hy- 

 drothecse of the colony, is here directed towards the internode, and 

 has its apocauline margin produced into a sharp, slightly incurved 

 tooth, and th.e internode with its pair of opposite liydrothecse has 

 the V-shaped form which is so general among the Sertularise, the 

 proximal portion rapidly narrowing to its point of union with the 

 internode which precedes it. Indeed, the resemblance of each 

 internode with, its pair of hydrothecse in this part of the colony 

 to those of the Sertularia operculata of our own coasts is singu- 

 larly striking. 



Amid systematic characters pointing in so many different 

 directions, it would seem difficult to decide on the true generic 

 position of our Hydroid. It may possibly be urged that this 

 singular combination of characters would justify its reference to 

 an entirely new genus ; and, indeed, if we could be sure that the 

 features thus presented were constant, this would perhaps be the 

 proper course to adopt. Until, however, the examination of a 

 greater number of specimens shall aiford evidence of the constant 

 recurrence of the same combination of characters, I believe it 

 will be better to refer it to one of the three genera represented 

 by it ; and as the characters of Thuiaria appear to be predo- 

 minant, I shall content myself with regarding it simply as a very 

 aberrant species of that genus. 



