HYDROIDA. BALE. 293 



Billard, is another form with the same characteristic. I 

 fully agree with Billard that these genera (except perhaps 

 Antennella) are entirely superfluous. 



The presence of an intrathecal ridge in Plumularia and its 

 immediate allies is, though not nearly so frequent as in the 

 Statoplea, by no means so exceptional as some observers 

 have suggested, and it occurs on both the adcauline and the 

 abcauline sides of the hydro theca. 1 An abcauline ridge, of 

 the same type as that of Lytocarpus philippinus, is found in 

 P. diaphragmata and P. jedani, Billard, and the same form 

 occurs in P. balei, Bartlett, as well as in Kirchenpaueria 

 mirabilis and K. producta, which Billard includes in Plumu- 

 laria. In all these cases the ridge is a distinct shelf extending 

 across a large portion of the hydrothecal cavity. In the 

 peculiar group which includes P. asymmetrica and P. alata 

 we find these two species possessing an abcauline ridge less 

 developed than that of the group last mentioned. In P. 

 habereri var. mediolineata, Billard, occurs the first slight 

 rudiment of the ridge which in the allied P. asymmetrica 

 attains a fuller development. Among the species in which 

 the ridge is adcauline is P. filicaulis ; in one form it is rudi- 

 mentary, in another it extends nearly half through the 

 hydrotheca, and it may be noted that though on the adcauline 

 side, it is not in that portion of the hydrotheca which is in 

 contact with the internode, but in the free portion. P. 

 lucerna, Mulder and Trebilcock, is said to have a similar 

 ridge ; I doubt, however, whether this species is distinct 

 from P. filicaulis. In P. goldsteini the adcauline ridge is 

 rudimentary. In the section Monotheca I have found only 

 the adcauline ridge, which springs from a point close to the 

 termination of the hydrocladium. In P. obliqua it is rudi- 

 mentary ; in the form which I have called var. robusta it is 

 somewhat more developed. Again, in P. compressa it is 

 very narrow, while in the closely allied P. australis it extends 

 half through the hydrotheca, and in P. spinulosa it is still 

 wider. 



As a guide to the affinities of the species I cannot attach 

 any importance to the presence of the intrathecal ridge, since 

 we often find it present in one species while another, evidently 

 very nearly allied, may have no trace of it. Such instances 

 are P. asymmetrica, Bale, and P. hertwigi, Stechow, among 

 the species with bilobed hydrothecse ; and P. balei, Bartlett, 



1. I use the terms " adcauline " and " abcauline " instead of " posterior" 

 and " anterior," as there are instances, both in the Statoplea and the 

 Eleutheroplea, where the adcauline ridge cannot with propriety be 

 -described as posterior. 



