HYDROIDA. BALE. 263 



S. insignis is said by Thompson to have " about six ' 

 teeth. 



S. huttoni is described as having " seven or eight " teeth. 



S. crenata has ten teeth, more uniform in size than those 

 of most of the preceding species. There is a median tooth 

 on both the adcauline and the abcauline borders, and four 

 on each side, the median teeth, however, are shorter than 

 the rest, and sometimes tend to become obsolete. 



Four of the species in the foregoing list agree in having 

 a similar habit, not, I think, found elsewhere among the 

 members of the old genus Sertularia. The pinnse are in 

 pairs and opposite, and associated with this character is 

 the unusual feature that the hydrothecae on the rachis are 

 opposite, while those on the pinnse are subalternate. They 

 have the further character in common that the first hydro- 

 thecal internode on each pinna bears a single hydrgtheca 

 only, which is on the lower side. These species are S. insignis, 

 S. acanthostoma, S. huttoni, and 8. billardi. I think it very 

 probable, however, that S. insignis and S. huttoni, which are 

 respectively recorded from Australia and New Zealand, are 

 one and the same. 



S. pluridentata is, in the form of the hydrothecae, very 

 nearly allied to S. acanthostoma, but the pinnse are irregular, 

 not opposite. 



In S. crenata the pinnae are distinctly alternate. In the 

 type specimen the rachis is divided alternately into inter- 

 nodes which support a single pinna with its axillary hydro- 

 theca, or a pair of hydrothecae only ; these obviously represent 

 the ordinary Sertularian internode (as found in S. elongata), 

 divided into two by a secondary node. The hydro thecse on 

 the pinnae are subalternate, and, as in the oppositely- 

 branched species, the first hydrothecal internode on each 

 supports but one hydrotheca, on the lower side. 



In the original specimen of S. crenata the teeth were all 

 well developed, except that the two median ones were some- 

 times much reduced. But I have a specimen from an un- 

 known locality, which I am doubtful whether to regard 

 as a variety of S. crenata or as a merely abnormal colony, in 

 which all the teeth are either much reduced, being indicated 

 mostly only by slight undulations of the border, or are 

 totally absent. In one or two hydrothecae there are four 

 of these undulations on each side, representing the eight 

 lateral teeth of the normal form, and the median teeth are 

 feebly indicated. In most cases, however, the latter are 

 wholly wanting, and the lateral sinuations are very slight 



